/*      $NetBSD: kern_lwp.c,v 1.271 2026/01/04 01:35:44 riastradh Exp $ */

/*-
 * Copyright (c) 2001, 2006, 2007, 2008, 2009, 2019, 2020, 2023
 *     The NetBSD Foundation, Inc.
 * All rights reserved.
 *
 * This code is derived from software contributed to The NetBSD Foundation
 * by Nathan J. Williams, and Andrew Doran.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

/*
 * Overview
 *
 *      Lightweight processes (LWPs) are the basic unit or thread of
 *      execution within the kernel.  The core state of an LWP is described
 *      by "struct lwp", also known as lwp_t.
 *
 *      Each LWP is contained within a process (described by "struct proc"),
 *      Every process contains at least one LWP, but may contain more.  The
 *      process describes attributes shared among all of its LWPs such as a
 *      private address space, global execution state (stopped, active,
 *      zombie, ...), signal disposition and so on.  On a multiprocessor
 *      machine, multiple LWPs be executing concurrently in the kernel.
 *
 * Execution states
 *
 *      At any given time, an LWP has overall state that is described by
 *      lwp::l_stat.  The states are broken into two sets below.  The first
 *      set is guaranteed to represent the absolute, current state of the
 *      LWP:
 *
 *      LSONPROC
 *
 *              On processor: the LWP is executing on a CPU, either in the
 *              kernel or in user space.
 *
 *      LSRUN
 *
 *              Runnable: the LWP is parked on a run queue, and may soon be
 *              chosen to run by an idle processor, or by a processor that
 *              has been asked to preempt a currently running but lower
 *              priority LWP.
 *
 *      LSIDL
 *
 *              Idle: the LWP has been created but has not yet executed, or
 *              it has ceased executing a unit of work and is waiting to be
 *              started again.  This state exists so that the LWP can occupy
 *              a slot in the process & PID table, but without having to
 *              worry about being touched; lookups of the LWP by ID will
 *              fail while in this state.  The LWP will become visible for
 *              lookup once its state transitions further.  Some special
 *              kernel threads also (ab)use this state to indicate that they
 *              are idle (soft interrupts and idle LWPs).
 *
 *      LSSUSPENDED:
 *
 *              Suspended: the LWP has had its execution suspended by
 *              another LWP in the same process using the _lwp_suspend()
 *              system call.  User-level LWPs also enter the suspended
 *              state when the system is shutting down.
 *
 *      The second set represent a "statement of intent" on behalf of the
 *      LWP.  The LWP may in fact be executing on a processor, may be
 *      sleeping or idle. It is expected to take the necessary action to
 *      stop executing or become "running" again within a short timeframe.
 *      The LP_RUNNING flag in lwp::l_pflag indicates that an LWP is running.
 *      Importantly, it indicates that its state is tied to a CPU.
 *
 *      LSZOMB:
 *
 *              Dead or dying: the LWP has released most of its resources
 *              and is about to switch away into oblivion, or has already
 *              switched away.  When it switches away, its few remaining
 *              resources can be collected.
 *
 *      LSSLEEP:
 *
 *              Sleeping: the LWP has entered itself onto a sleep queue, and
 *              has switched away or will switch away shortly to allow other
 *              LWPs to run on the CPU.
 *
 *      LSSTOP:
 *
 *              Stopped: the LWP has been stopped as a result of a job
 *              control signal, or as a result of the ptrace() interface. 
 *
 *              Stopped LWPs may run briefly within the kernel to handle
 *              signals that they receive, but will not return to user space
 *              until their process' state is changed away from stopped. 
 *
 *              Single LWPs within a process can not be set stopped
 *              selectively: all actions that can stop or continue LWPs
 *              occur at the process level.
 *
 * State transitions
 *
 *      Note that the LSSTOP state may only be set when returning to
 *      user space in userret(), or when sleeping interruptably.  The
 *      LSSUSPENDED state may only be set in userret().  Before setting
 *      those states, we try to ensure that the LWPs will release all
 *      locks that they hold, and at a minimum try to ensure that the
 *      LWP can be set runnable again by a signal.
 *
 *      LWPs may transition states in the following ways:
 *
 *       RUN -------> ONPROC            ONPROC -----> RUN
 *                                                  > SLEEP
 *                                                  > STOPPED
 *                                                  > SUSPENDED
 *                                                  > ZOMB
 *                                                  > IDL (special cases)
 *
 *       STOPPED ---> RUN               SUSPENDED --> RUN
 *                  > SLEEP
 *
 *       SLEEP -----> ONPROC            IDL --------> RUN
 *                  > RUN                           > SUSPENDED
 *                  > STOPPED                       > STOPPED
 *                                                  > ONPROC (special cases)
 *
 *      Some state transitions are only possible with kernel threads (eg
 *      ONPROC -> IDL) and happen under tightly controlled circumstances
 *      free of unwanted side effects.
 *
 * Migration
 *
 *      Migration of threads from one CPU to another could be performed
 *      internally by the scheduler via sched_takecpu() or sched_catchlwp()
 *      functions.  The universal lwp_migrate() function should be used for
 *      any other cases.  Subsystems in the kernel must be aware that CPU
 *      of LWP may change, while it is not locked.
 *
 * Locking
 *
 *      The majority of fields in 'struct lwp' are covered by a single,
 *      general spin lock pointed to by lwp::l_mutex.  The locks covering
 *      each field are documented in sys/lwp.h.
 *
 *      State transitions must be made with the LWP's general lock held,
 *      and may cause the LWP's lock pointer to change.  Manipulation of
 *      the general lock is not performed directly, but through calls to
 *      lwp_lock(), lwp_unlock() and others.  It should be noted that the
 *      adaptive locks are not allowed to be released while the LWP's lock
 *      is being held (unlike for other spin-locks).
 *
 *      States and their associated locks:
 *
 *      LSIDL, LSONPROC, LSZOMB, LSSUSPENDED:
 *
 *              Always covered by spc_lwplock, which protects LWPs not
 *              associated with any other sync object.  This is a per-CPU
 *              lock and matches lwp::l_cpu.
 *
 *      LSRUN:
 *
 *              Always covered by spc_mutex, which protects the run queues.
 *              This is a per-CPU lock and matches lwp::l_cpu.
 *
 *      LSSLEEP:
 *
 *              Covered by a lock associated with the sleep queue (sometimes
 *              a turnstile sleep queue) that the LWP resides on.  This can
 *              be spc_lwplock for SOBJ_SLEEPQ_NULL (an "untracked" sleep).
 *
 *      LSSTOP:
 *
 *              If the LWP was previously sleeping (l_wchan != NULL), then
 *              l_mutex references the sleep queue lock.  If the LWP was
 *              runnable or on the CPU when halted, or has been removed from
 *              the sleep queue since halted, then the lock is spc_lwplock.
 *
 *      The lock order is as follows:
 *
 *              sleepq -> turnstile -> spc_lwplock -> spc_mutex
 *
 *      Each process has a scheduler state lock (proc::p_lock), and a
 *      number of counters on LWPs and their states: p_nzlwps, p_nrlwps, and
 *      so on.  When an LWP is to be entered into or removed from one of the
 *      following states, p_lock must be held and the process wide counters
 *      adjusted:
 *
 *              LSIDL, LSZOMB, LSSTOP, LSSUSPENDED
 *
 *      (But not always for kernel threads.  There are some special cases
 *      as mentioned above: soft interrupts, and the idle loops.)
 *
 *      Note that an LWP is considered running or likely to run soon if in
 *      one of the following states.  This affects the value of p_nrlwps:
 *
 *              LSRUN, LSONPROC, LSSLEEP
 *
 *      p_lock does not need to be held when transitioning among these
 *      three states, hence p_lock is rarely taken for state transitions.
 */

#include <sys/cdefs.h>
__KERNEL_RCSID(0, "$NetBSD: kern_lwp.c,v 1.271 2026/01/04 01:35:44 riastradh Exp $");

#include "opt_ddb.h"
#include "opt_lockdebug.h"
#include "opt_dtrace.h"

#define _LWP_API_PRIVATE

#include <sys/param.h>

#include <sys/atomic.h>
#include <sys/cprng.h>
#include <sys/cpu.h>
#include <sys/dtrace_bsd.h>
#include <sys/filedesc.h>
#include <sys/fstrans.h>
#include <sys/futex.h>
#include <sys/intr.h>
#include <sys/kauth.h>
#include <sys/kcov.h>
#include <sys/kmem.h>
#include <sys/lockdebug.h>
#include <sys/lwpctl.h>
#include <sys/msan.h>
#include <sys/pool.h>
#include <sys/proc.h>
#include <sys/pset.h>
#include <sys/psref.h>
#include <sys/ptrace.h>
#include <sys/sdt.h>
#include <sys/sleepq.h>
#include <sys/syncobj.h>
#include <sys/syscall_stats.h>
#include <sys/syscallargs.h>
#include <sys/sysctl.h>
#include <sys/systm.h>
#include <sys/uidinfo.h>
#include <sys/xcall.h>

#include <uvm/uvm_extern.h>
#include <uvm/uvm_object.h>

static pool_cache_t     lwp_cache       __read_mostly;
struct lwplist          alllwp          __cacheline_aligned;

static int              lwp_ctor(void *, void *, int);
static void             lwp_dtor(void *, void *);

/* DTrace proc provider probes */
SDT_PROVIDER_DEFINE(proc);

SDT_PROBE_DEFINE1(proc, kernel, , lwp__create, "struct lwp *");
SDT_PROBE_DEFINE1(proc, kernel, , lwp__start, "struct lwp *");
SDT_PROBE_DEFINE1(proc, kernel, , lwp__exit, "struct lwp *");

struct turnstile turnstile0 __cacheline_aligned;
struct lwp lwp0 __aligned(MIN_LWP_ALIGNMENT) = {
#ifdef LWP0_CPU_INFO
        .l_cpu = LWP0_CPU_INFO,
#endif
#ifdef LWP0_MD_INITIALIZER
        .l_md = LWP0_MD_INITIALIZER,
#endif
        .l_proc = &proc0,
        .l_lid = 0,             /* we own proc0's slot in the pid table */
        .l_flag = LW_SYSTEM,
        .l_stat = LSONPROC,
        .l_ts = &turnstile0,
        .l_syncobj = &sched_syncobj,
        .l_refcnt = 0,
        .l_priority = PRI_USER + NPRI_USER - 1,
        .l_inheritedprio = -1,
        .l_class = SCHED_OTHER,
        .l_psid = PS_NONE,
        .l_pi_lenders = SLIST_HEAD_INITIALIZER(&lwp0.l_pi_lenders),
        .l_name = __UNCONST("swapper"),
        .l_fd = &filedesc0,
};

static int
lwp_maxlwp(void)
{
        /* Assume 1 LWP per 1MiB. */
        uint64_t lwps_per = ctob(physmem) / (1024 * 1024);

        return MAX(MIN(MAXMAXLWP, lwps_per), MAXLWP);
}

static int sysctl_kern_maxlwp(SYSCTLFN_PROTO);

/*
 * sysctl helper routine for kern.maxlwp. Ensures that the new
 * values are not too low or too high.
 */
static int
sysctl_kern_maxlwp(SYSCTLFN_ARGS)
{
        int error, nmaxlwp;
        struct sysctlnode node;

        nmaxlwp = maxlwp;
        node = *rnode;
        node.sysctl_data = &nmaxlwp;
        error = sysctl_lookup(SYSCTLFN_CALL(&node));
        if (error || newp == NULL)
                return error;

        if (nmaxlwp < 0 || nmaxlwp >= MAXMAXLWP)
                return SET_ERROR(EINVAL);
        if (nmaxlwp > lwp_maxlwp())
                return SET_ERROR(EINVAL);
        maxlwp = nmaxlwp;

        return 0;
}

static void
sysctl_kern_lwp_setup(void)
{
        sysctl_createv(NULL, 0, NULL, NULL,
                       CTLFLAG_PERMANENT|CTLFLAG_READWRITE,
                       CTLTYPE_INT, "maxlwp",
                       SYSCTL_DESCR("Maximum number of simultaneous threads"),
                       sysctl_kern_maxlwp, 0, NULL, 0,
                       CTL_KERN, CTL_CREATE, CTL_EOL);
}

void
lwpinit(void)
{

        LIST_INIT(&alllwp);
        lwpinit_specificdata();
        /*
         * Provide a barrier to ensure that all mutex_oncpu() and rw_oncpu()
         * calls will exit before memory of LWPs is returned to the pool, where
         * KVA of LWP structure might be freed and re-used for other purposes.
         * Kernel preemption is disabled around mutex_oncpu() and rw_oncpu()
         * callers, therefore a regular passive serialization barrier will
         * do the job.
         */
        lwp_cache = pool_cache_init(sizeof(lwp_t), MIN_LWP_ALIGNMENT, 0,
            PR_PSERIALIZE, "lwppl", NULL, IPL_NONE, lwp_ctor, lwp_dtor, NULL);

        maxlwp = lwp_maxlwp();
        sysctl_kern_lwp_setup();
}

void
lwp0_init(void)
{
        struct lwp *l = &lwp0;

        KASSERT((void *)uvm_lwp_getuarea(l) != NULL);

        LIST_INSERT_HEAD(&alllwp, l, l_list);

        callout_init(&l->l_timeout_ch, CALLOUT_MPSAFE);
        callout_setfunc(&l->l_timeout_ch, sleepq_timeout, l);
        cv_init(&l->l_sigcv, "sigwait");
        cv_init(&l->l_waitcv, "vfork");

        l->l_cred = kauth_cred_hold(proc0.p_cred);

        kdtrace_thread_ctor(NULL, l);
        lwp_initspecific(l);

        SYSCALL_TIME_LWP_INIT(l);
}

/*
 * Initialize the non-zeroed portion of an lwp_t.
 */
static int
lwp_ctor(void *arg, void *obj, int flags)
{
        lwp_t *l = obj;

        l->l_stat = LSIDL;
        l->l_cpu = curcpu();
        l->l_mutex = l->l_cpu->ci_schedstate.spc_lwplock;
        l->l_ts = kmem_alloc(sizeof(*l->l_ts), flags == PR_WAITOK ?
            KM_SLEEP : KM_NOSLEEP);

        if (l->l_ts == NULL) {
                return SET_ERROR(ENOMEM);
        } else {
                turnstile_ctor(l->l_ts);
                return 0;
        }
}

static void
lwp_dtor(void *arg, void *obj)
{
        lwp_t *l = obj;

        /*
         * The value of l->l_cpu must still be valid at this point.
         */
        KASSERT(l->l_cpu != NULL);

        /*
         * We can't return turnstile0 to the pool (it didn't come from it),
         * so if it comes up just drop it quietly and move on.
         */
        if (l->l_ts != &turnstile0)
                kmem_free(l->l_ts, sizeof(*l->l_ts));
}

/*
 * Set an LWP suspended.
 *
 * Must be called with p_lock held, and the LWP locked.  Will unlock the
 * LWP before return.
 */
int
lwp_suspend(struct lwp *curl, struct lwp *t)
{
        int error;

        KASSERT(mutex_owned(t->l_proc->p_lock));
        KASSERT(lwp_locked(t, NULL));

        KASSERT(curl != t || curl->l_stat == LSONPROC);

        /*
         * If the current LWP has been told to exit, we must not suspend anyone
         * else or deadlock could occur.  We won't return to userspace.
         */
        if ((curl->l_flag & (LW_WEXIT | LW_WCORE)) != 0) {
                lwp_unlock(t);
                return SET_ERROR(EDEADLK);
        }

        if ((t->l_flag & LW_DBGSUSPEND) != 0) {
                lwp_unlock(t);
                return 0;
        }

        error = 0;

        switch (t->l_stat) {
        case LSRUN:
        case LSONPROC:
                t->l_flag |= LW_WSUSPEND;
                lwp_need_userret(t);
                lwp_unlock(t);
                break;

        case LSSLEEP:
                t->l_flag |= LW_WSUSPEND;
                lwp_need_userret(t);

                /*
                 * Kick the LWP and try to get it to the kernel boundary
                 * so that it will release any locks that it holds.
                 * setrunnable() will release the lock.
                 */
                if ((t->l_flag & LW_SINTR) != 0)
                        setrunnable(t);
                else
                        lwp_unlock(t);
                break;

        case LSSUSPENDED:
                lwp_unlock(t);
                break;

        case LSSTOP:
                t->l_flag |= LW_WSUSPEND;
                lwp_need_userret(t);
                setrunnable(t);
                break;

        case LSIDL:
        case LSZOMB:
                error = SET_ERROR(EINTR); /* It's what Solaris does..... */
                lwp_unlock(t);
                break;
        }

        return (error);
}

/*
 * Restart a suspended LWP.
 *
 * Must be called with p_lock held, and the LWP locked.  Will unlock the
 * LWP before return.
 */
void
lwp_continue(struct lwp *l)
{

        KASSERT(mutex_owned(l->l_proc->p_lock));
        KASSERT(lwp_locked(l, NULL));

        /* If rebooting or not suspended, then just bail out. */
        if ((l->l_flag & LW_WREBOOT) != 0) {
                lwp_unlock(l);
                return;
        }

        l->l_flag &= ~LW_WSUSPEND;

        if (l->l_stat != LSSUSPENDED || (l->l_flag & LW_DBGSUSPEND) != 0) {
                lwp_unlock(l);
                return;
        }

        /* setrunnable() will release the lock. */
        setrunnable(l);
}

/*
 * Restart a stopped LWP.
 *
 * Must be called with p_lock held, and the LWP NOT locked.  Will unlock the
 * LWP before return.
 */
void
lwp_unstop(struct lwp *l)
{
        struct proc *p = l->l_proc;

        KASSERT(mutex_owned(&proc_lock));
        KASSERT(mutex_owned(p->p_lock));

        lwp_lock(l);

        KASSERT((l->l_flag & LW_DBGSUSPEND) == 0);

        /* If not stopped, then just bail out. */
        if (l->l_stat != LSSTOP) {
                lwp_unlock(l);
                return;
        }

        p->p_stat = SACTIVE;
        p->p_sflag &= ~PS_STOPPING;

        if (!p->p_waited)
                p->p_pptr->p_nstopchild--;

        if (l->l_wchan == NULL) {
                /* setrunnable() will release the lock. */
                setrunnable(l);
        } else if (p->p_xsig && (l->l_flag & LW_SINTR) != 0) {
                /* setrunnable() so we can receive the signal */
                setrunnable(l);
        } else {
                l->l_stat = LSSLEEP;
                p->p_nrlwps++;
                lwp_unlock(l);
        }
}

/*
 * Wait for an LWP within the current process to exit.  If 'lid' is
 * non-zero, we are waiting for a specific LWP.
 *
 * Must be called with p->p_lock held.
 */
int
lwp_wait(struct lwp *l, lwpid_t lid, lwpid_t *departed, bool exiting)
{
        const lwpid_t curlid = l->l_lid;
        proc_t *p = l->l_proc;
        lwp_t *l2, *next;
        int error;

        KASSERT(mutex_owned(p->p_lock));

        p->p_nlwpwait++;
        l->l_waitingfor = lid;

        for (;;) {
                int nfound;

                /*
                 * Avoid a race between exit1() and sigexit(): if the
                 * process is dumping core, then we need to bail out: call
                 * into lwp_userret() where we will be suspended until the
                 * deed is done.
                 */
                if ((p->p_sflag & PS_WCORE) != 0) {
                        mutex_exit(p->p_lock);
                        lwp_userret(l);
                        KASSERT(false);
                }

                /*
                 * First off, drain any detached LWP that is waiting to be
                 * reaped.
                 */
                if ((l2 = p->p_zomblwp) != NULL) {
                        p->p_zomblwp = NULL;
                        lwp_free(l2, false, false);/* releases proc mutex */
                        mutex_enter(p->p_lock);
                        continue;
                }

                /*
                 * Now look for an LWP to collect.  If the whole process is
                 * exiting, count detached LWPs as eligible to be collected,
                 * but don't drain them here.
                 */
                nfound = 0;
                error = 0;

                /*
                 * If given a specific LID, go via pid_table and make sure
                 * it's not detached.
                 */
                if (lid != 0) {
                        l2 = proc_find_lwp(p, lid);
                        if (l2 == NULL) {
                                error = SET_ERROR(ESRCH);
                                break;
                        }
                        KASSERT(l2->l_lid == lid);
                        if ((l2->l_prflag & LPR_DETACHED) != 0) {
                                error = SET_ERROR(EINVAL);
                                break;
                        }
                } else {
                        l2 = LIST_FIRST(&p->p_lwps);
                }
                for (; l2 != NULL; l2 = next) {
                        next = (lid != 0 ? NULL : LIST_NEXT(l2, l_sibling));

                        /*
                         * If a specific wait and the target is waiting on
                         * us, then avoid deadlock.  This also traps LWPs
                         * that try to wait on themselves.
                         *
                         * Note that this does not handle more complicated
                         * cycles, like: t1 -> t2 -> t3 -> t1.  The process
                         * can still be killed so it is not a major problem.
                         */
                        if (l2->l_lid == lid && l2->l_waitingfor == curlid) {
                                error = SET_ERROR(EDEADLK);
                                break;
                        }
                        if (l2 == l)
                                continue;
                        if ((l2->l_prflag & LPR_DETACHED) != 0) {
                                nfound += exiting;
                                continue;
                        }
                        if (lid != 0) {
                                /*
                                 * Mark this LWP as the first waiter, if there
                                 * is no other.
                                 */
                                if (l2->l_waiter == 0)
                                        l2->l_waiter = curlid;
                        } else if (l2->l_waiter != 0) {
                                /*
                                 * It already has a waiter - so don't
                                 * collect it.  If the waiter doesn't
                                 * grab it we'll get another chance
                                 * later.
                                 */
                                nfound++;
                                continue;
                        }
                        nfound++;

                        /* No need to lock the LWP in order to see LSZOMB. */
                        if (l2->l_stat != LSZOMB)
                                continue;

                        /*
                         * We're no longer waiting.  Reset the "first waiter"
                         * pointer on the target, in case it was us.
                         */
                        l->l_waitingfor = 0;
                        l2->l_waiter = 0;
                        p->p_nlwpwait--;
                        if (departed)
                                *departed = l2->l_lid;
                        sched_lwp_collect(l2);

                        /* lwp_free() releases the proc lock. */
                        lwp_free(l2, false, false);
                        mutex_enter(p->p_lock);
                        return 0;
                }

                if (error != 0)
                        break;
                if (nfound == 0) {
                        error = SET_ERROR(ESRCH);
                        break;
                }

                /*
                 * Note: since the lock will be dropped, need to restart on
                 * wakeup to run all LWPs again, e.g. there may be new LWPs.
                 */
                if (exiting) {
                        KASSERT(p->p_nlwps > 1);
                        error = cv_timedwait(&p->p_lwpcv, p->p_lock, 1);
                        break;
                }

                /*
                 * Break out if all LWPs are in _lwp_wait().  There are
                 * other ways to hang the process with _lwp_wait(), but the
                 * sleep is interruptable so little point checking for them.
                 */
                if (p->p_nlwpwait == p->p_nlwps) {
                        error = SET_ERROR(EDEADLK);
                        break;
                }

                /*
                 * Sit around and wait for something to happen.  We'll be 
                 * awoken if any of the conditions examined change: if an
                 * LWP exits, is collected, or is detached.
                 */
                if ((error = cv_wait_sig(&p->p_lwpcv, p->p_lock)) != 0)
                        break;
        }

        /*
         * We didn't find any LWPs to collect, we may have received a 
         * signal, or some other condition has caused us to bail out.
         *
         * If waiting on a specific LWP, clear the waiters marker: some
         * other LWP may want it.  Then, kick all the remaining waiters
         * so that they can re-check for zombies and for deadlock.
         */
        if (lid != 0) {
                l2 = proc_find_lwp(p, lid);
                KASSERT(l2 == NULL || l2->l_lid == lid);

                if (l2 != NULL && l2->l_waiter == curlid)
                        l2->l_waiter = 0;
        }
        p->p_nlwpwait--;
        l->l_waitingfor = 0;
        cv_broadcast(&p->p_lwpcv);

        return error;
}

/*
 * Create a new LWP within process 'p2', using LWP 'l1' as a template.
 * The new LWP is created in state LSIDL and must be set running,
 * suspended, or stopped by the caller.
 */
int
lwp_create(lwp_t *l1, proc_t *p2, vaddr_t uaddr, int flags,
    void *stack, size_t stacksize, void (*func)(void *), void *arg,
    lwp_t **rnewlwpp, int sclass, const sigset_t *sigmask,
    const stack_t *sigstk)
{
        struct lwp *l2;

        KASSERT(l1 == curlwp || l1->l_proc == &proc0);

        /*
         * Enforce limits, excluding the first lwp and kthreads.  We must
         * use the process credentials here when adjusting the limit, as
         * they are what's tied to the accounting entity.  However for
         * authorizing the action, we'll use the LWP's credentials.
         */
        mutex_enter(p2->p_lock);
        if (p2->p_nlwps != 0 && p2 != &proc0) {
                uid_t uid = kauth_cred_getuid(p2->p_cred);
                int count = chglwpcnt(uid, 1);
                if (__predict_false(count >
                    p2->p_rlimit[RLIMIT_NTHR].rlim_cur)) {
                        if (kauth_authorize_process(l1->l_cred,
                            KAUTH_PROCESS_RLIMIT, p2,
                            KAUTH_ARG(KAUTH_REQ_PROCESS_RLIMIT_BYPASS),
                            &p2->p_rlimit[RLIMIT_NTHR], KAUTH_ARG(RLIMIT_NTHR))
                            != 0) {
                                (void)chglwpcnt(uid, -1);
                                mutex_exit(p2->p_lock);
                                return SET_ERROR(EAGAIN);
                        }
                }
        }

        /*
         * First off, reap any detached LWP waiting to be collected.
         * We can re-use its LWP structure and turnstile.
         */
        if ((l2 = p2->p_zomblwp) != NULL) {
                p2->p_zomblwp = NULL;
                lwp_free(l2, true, false);
                /* p2 now unlocked by lwp_free() */
                KASSERT(l2->l_ts != NULL);
                KASSERT(l2->l_inheritedprio == -1);
                KASSERT(SLIST_EMPTY(&l2->l_pi_lenders));
                memset(&l2->l_startzero, 0, sizeof(*l2) -
                    offsetof(lwp_t, l_startzero));
        } else {
                mutex_exit(p2->p_lock);
                l2 = pool_cache_get(lwp_cache, PR_WAITOK);
                memset(&l2->l_startzero, 0, sizeof(*l2) -
                    offsetof(lwp_t, l_startzero));
                SLIST_INIT(&l2->l_pi_lenders);
        }

        /*
         * Because of lockless lookup via pid_table, the LWP can be locked
         * and inspected briefly even after it's freed, so a few fields are
         * kept stable.
         */
        KASSERT(l2->l_stat == LSIDL);
        KASSERT(l2->l_cpu != NULL);
        KASSERT(l2->l_ts != NULL);
        KASSERT(l2->l_mutex == l2->l_cpu->ci_schedstate.spc_lwplock);

        l2->l_proc = p2;
        l2->l_refcnt = 0;
        l2->l_class = sclass;

        /*
         * Allocate a process ID for this LWP.  We need to do this now
         * while we can still unwind if it fails.  Because we're marked
         * as LSIDL, no lookups by the ID will succeed.
         *
         * N.B. this will always succeed for the first LWP in a process,
         * because proc_alloc_lwpid() will usurp the slot.  Also note
         * that l2->l_proc MUST be valid so that lookups of the proc
         * will succeed, even if the LWP itself is not visible.
         */
        if (__predict_false(proc_alloc_lwpid(p2, l2) == -1)) {
                pool_cache_put(lwp_cache, l2);
                return SET_ERROR(EAGAIN);
        }

        /*
         * If vfork(), we want the LWP to run fast and on the same CPU
         * as its parent, so that it can reuse the VM context and cache
         * footprint on the local CPU.
         */
        l2->l_boostpri = ((flags & LWP_VFORK) ? PRI_KERNEL : PRI_USER);
        l2->l_priority = l1->l_priority;
        l2->l_inheritedprio = -1;
        l2->l_protectprio = -1;
        l2->l_auxprio = -1;
        l2->l_flag = 0;
        l2->l_pflag = LP_MPSAFE;
        TAILQ_INIT(&l2->l_ld_locks);
        l2->l_psrefs = 0;
        kmsan_lwp_alloc(l2);

        /*
         * For vfork, borrow parent's lwpctl context if it exists.
         * This also causes us to return via lwp_userret.
         */
        if (flags & LWP_VFORK && l1->l_lwpctl) {
                l2->l_lwpctl = l1->l_lwpctl;
                l2->l_flag |= LW_LWPCTL;
        }

        /*
         * If not the first LWP in the process, grab a reference to the
         * descriptor table.
         */
        l2->l_fd = p2->p_fd;
        if (p2->p_nlwps != 0) {
                KASSERT(l1->l_proc == p2);
                fd_hold(l2);
        } else {
                KASSERT(l1->l_proc != p2);
        }

        if (p2->p_flag & PK_SYSTEM) {
                /* Mark it as a system LWP. */
                l2->l_flag |= LW_SYSTEM;
        }

        kdtrace_thread_ctor(NULL, l2);
        lwp_initspecific(l2);
        sched_lwp_fork(l1, l2);
        callout_init(&l2->l_timeout_ch, CALLOUT_MPSAFE);
        callout_setfunc(&l2->l_timeout_ch, sleepq_timeout, l2);
        cv_init(&l2->l_sigcv, "sigwait");
        cv_init(&l2->l_waitcv, "vfork");
        l2->l_syncobj = &sched_syncobj;
        PSREF_DEBUG_INIT_LWP(l2);

        if (rnewlwpp != NULL)
                *rnewlwpp = l2;

        /*
         * PCU state needs to be saved before calling uvm_lwp_fork() so that
         * the MD cpu_lwp_fork() can copy the saved state to the new LWP.
         */
        pcu_save_all(l1);
#if PCU_UNIT_COUNT > 0
        l2->l_pcu_valid = l1->l_pcu_valid;
#endif

        uvm_lwp_setuarea(l2, uaddr);
        uvm_lwp_fork(l1, l2, stack, stacksize, func, (arg != NULL) ? arg : l2);

        mutex_enter(p2->p_lock);
        l2->l_cred = kauth_cred_hold(p2->p_cred);
        if ((flags & LWP_DETACHED) != 0) {
                l2->l_prflag = LPR_DETACHED;
                p2->p_ndlwps++;
        } else
                l2->l_prflag = 0;

        if (l1->l_proc == p2) {
                /*
                 * These flags are set while p_lock is held.  Copy with
                 * p_lock held too, so the LWP doesn't sneak into the
                 * process without them being set.
                 */
                l2->l_flag |= (l1->l_flag & (LW_WEXIT | LW_WREBOOT | LW_WCORE));
        } else {
                /* fork(): pending core/exit doesn't apply to child. */
                l2->l_flag |= (l1->l_flag & LW_WREBOOT);
        }

        l2->l_sigstk = *sigstk;
        l2->l_sigmask = *sigmask;
        TAILQ_INIT(&l2->l_sigpend.sp_info);
        sigemptyset(&l2->l_sigpend.sp_set);
        LIST_INSERT_HEAD(&p2->p_lwps, l2, l_sibling);
        p2->p_nlwps++;
        p2->p_nrlwps++;

        KASSERT(l2->l_affinity == NULL);

        /* Inherit the affinity mask. */
        if (l1->l_affinity) {
                /*
                 * Note that we hold the state lock while inheriting
                 * the affinity to avoid race with sched_setaffinity().
                 */
                lwp_lock(l1);
                if (l1->l_affinity) {
                        kcpuset_use(l1->l_affinity);
                        l2->l_affinity = l1->l_affinity;
                }
                lwp_unlock(l1);
        }

        /* Ensure a trip through lwp_userret() if needed. */
        if ((l2->l_flag & LW_USERRET) != 0) {
                lwp_need_userret(l2);
        }

        /* This marks the end of the "must be atomic" section. */
        mutex_exit(p2->p_lock);

        SDT_PROBE(proc, kernel, , lwp__create, l2, 0, 0, 0, 0);

        mutex_enter(&proc_lock);
        LIST_INSERT_HEAD(&alllwp, l2, l_list);
        /* Inherit a processor-set */
        l2->l_psid = l1->l_psid;
        mutex_exit(&proc_lock);

        SYSCALL_TIME_LWP_INIT(l2);

        if (p2->p_emul->e_lwp_fork)
                (*p2->p_emul->e_lwp_fork)(l1, l2);

        return (0);
}

/*
 * Set a new LWP running.  If the process is stopping, then the LWP is
 * created stopped.
 */
void
lwp_start(lwp_t *l, int flags)
{
        proc_t *p = l->l_proc;

        mutex_enter(p->p_lock);
        lwp_lock(l);
        KASSERT(l->l_stat == LSIDL);
        if ((flags & LWP_SUSPENDED) != 0) {
                /* It'll suspend itself in lwp_userret(). */
                l->l_flag |= LW_WSUSPEND;
                lwp_need_userret(l);
        }
        if (p->p_stat == SSTOP || (p->p_sflag & PS_STOPPING) != 0) {
                KASSERT(l->l_wchan == NULL);
                l->l_stat = LSSTOP;
                p->p_nrlwps--;
                lwp_unlock(l);
        } else {
                setrunnable(l);
                /* LWP now unlocked */
        }
        mutex_exit(p->p_lock);
}

/*
 * Called by MD code when a new LWP begins execution.  Must be called
 * with the previous LWP locked (so at splsched), or if there is no
 * previous LWP, at splsched.
 */
void
lwp_startup(struct lwp *prev, struct lwp *new_lwp)
{
        kmutex_t *lock;

        KASSERTMSG(new_lwp == curlwp, "l %p curlwp %p prevlwp %p", new_lwp, curlwp, prev);
        KASSERT(kpreempt_disabled());
        KASSERT(prev != NULL);
        KASSERT((prev->l_pflag & LP_RUNNING) != 0);
        KASSERT(curcpu()->ci_mtx_count == -2);

        /*
         * Immediately mark the previous LWP as no longer running and
         * unlock (to keep lock wait times short as possible).  If a
         * zombie, don't touch after clearing LP_RUNNING as it could be
         * reaped by another CPU.  Use atomic_store_release to ensure
         * this -- matches atomic_load_acquire in lwp_free.
         */
        lock = prev->l_mutex;
        if (__predict_false(prev->l_stat == LSZOMB)) {
                atomic_store_release(&prev->l_pflag,
                    prev->l_pflag & ~LP_RUNNING);
        } else {
                prev->l_pflag &= ~LP_RUNNING;
        }
        mutex_spin_exit(lock);

        /* Correct spin mutex count after mi_switch(). */
        curcpu()->ci_mtx_count = 0;

        /* Install new VM context. */
        if (__predict_true(new_lwp->l_proc->p_vmspace)) {
                pmap_activate(new_lwp);
        }

        /* We remain at IPL_SCHED from mi_switch() - reset it. */
        spl0();

        LOCKDEBUG_BARRIER(NULL, 0);
        SDT_PROBE(proc, kernel, , lwp__start, new_lwp, 0, 0, 0, 0);

        /* For kthreads, acquire kernel lock if not MPSAFE. */
        if (__predict_false((new_lwp->l_pflag & LP_MPSAFE) == 0)) {
                KERNEL_LOCK(1, new_lwp);
        }
}

/*
 * Exit an LWP.
 *
 * *** WARNING *** This can be called with (l != curlwp) in error paths.
 */
void
lwp_exit(struct lwp *l)
{
        struct proc *p = l->l_proc;
        struct lwp *l2;
        bool current;

        current = (l == curlwp);

        KASSERT(current || l->l_stat == LSIDL);
        KASSERT(current || l->l_target_cpu == NULL);
        KASSERT(p == curproc);

        SDT_PROBE(proc, kernel, , lwp__exit, l, 0, 0, 0, 0);

        /* Verify that we hold no locks; for DIAGNOSTIC check kernel_lock. */
        LOCKDEBUG_BARRIER(NULL, 0);
        KASSERTMSG(curcpu()->ci_biglock_count == 0, "kernel_lock leaked");

        /*
         * If we are the last live LWP in a process, we need to exit the
         * entire process.  We do so with an exit status of zero, because
         * it's a "controlled" exit, and because that's what Solaris does.
         *
         * We are not quite a zombie yet, but for accounting purposes we
         * must increment the count of zombies here.
         *
         * Note: the last LWP's specificdata will be deleted here.
         */
        mutex_enter(p->p_lock);
        if (p->p_nlwps - p->p_nzlwps == 1) {
                KASSERT(current == true);
                KASSERT(p != &proc0);
                exit1(l, 0, 0);
                /* NOTREACHED */
        }
        p->p_nzlwps++;

        /*
         * Perform any required thread cleanup.  Do this early so
         * anyone wanting to look us up with lwp_getref_lwpid() will
         * fail to find us before we become a zombie.
         *
         * N.B. this will unlock p->p_lock on our behalf.
         */
        lwp_thread_cleanup(l);

        if (p->p_emul->e_lwp_exit)
                (*p->p_emul->e_lwp_exit)(l);

        /* Drop filedesc reference. */
        fd_free();

        /* Release fstrans private data. */
        fstrans_lwp_dtor(l);

        /* Delete the specificdata while it's still safe to sleep. */
        lwp_finispecific(l);

        /*
         * Release our cached credentials.
         */
        kauth_cred_free(l->l_cred);
        callout_destroy(&l->l_timeout_ch);

        /*
         * If traced, report LWP exit event to the debugger.
         *
         * Remove the LWP from the global list.
         * Free its LID from the PID namespace if needed.
         */
        mutex_enter(&proc_lock);

        if ((p->p_slflag & (PSL_TRACED|PSL_TRACELWP_EXIT)) ==
            (PSL_TRACED|PSL_TRACELWP_EXIT)) {
                mutex_enter(p->p_lock);
                if (ISSET(p->p_sflag, PS_WEXIT)) {
                        mutex_exit(p->p_lock);
                        /*
                         * We are exiting, bail out without informing parent
                         * about a terminating LWP as it would deadlock.
                         */
                } else {
                        eventswitch(TRAP_LWP, PTRACE_LWP_EXIT, l->l_lid);
                        mutex_enter(&proc_lock);
                }
        }

        LIST_REMOVE(l, l_list);
        mutex_exit(&proc_lock);

        /*
         * Get rid of all references to the LWP that others (e.g. procfs)
         * may have, and mark the LWP as a zombie.  If the LWP is detached,
         * mark it waiting for collection in the proc structure.  Note that
         * before we can do that, we need to free any other dead, detached
         * LWP waiting to meet its maker.
         *
         * All conditions need to be observed upon under the same hold of
         * p_lock, because if the lock is dropped any of them can change.
         */
        mutex_enter(p->p_lock);
        for (;;) {
                if (lwp_drainrefs(l))
                        continue;
                if ((l->l_prflag & LPR_DETACHED) != 0) {
                        if ((l2 = p->p_zomblwp) != NULL) {
                                p->p_zomblwp = NULL;
                                lwp_free(l2, false, false);
                                /* proc now unlocked */
                                mutex_enter(p->p_lock);
                                continue;
                        }
                        p->p_zomblwp = l;
                }
                break;
        }

        /*
         * If we find a pending signal for the process and we have been
         * asked to check for signals, then we lose: arrange to have
         * all other LWPs in the process check for signals.
         */
        if ((l->l_flag & LW_PENDSIG) != 0 &&
            firstsig(&p->p_sigpend.sp_set) != 0) {
                LIST_FOREACH(l2, &p->p_lwps, l_sibling) {
                        lwp_lock(l2);
                        signotify(l2);
                        lwp_unlock(l2);
                }
        }

        /*
         * Release any PCU resources before becoming a zombie.
         */
        pcu_discard_all(l);

        lwp_lock(l);
        l->l_stat = LSZOMB;
        if (l->l_name != NULL) {
                strcpy(l->l_name, "(zombie)");
        }
        lwp_unlock(l);
        p->p_nrlwps--;
        if (l->l_lwpctl != NULL)
                l->l_lwpctl->lc_curcpu = LWPCTL_CPU_EXITED;
        mutex_exit(p->p_lock);
        cv_broadcast(&p->p_lwpcv);

        /*
         * We can no longer block.  At this point, lwp_free() may already
         * be gunning for us.  On a multi-CPU system, we may be off p_lwps.
         *
         * Free MD LWP resources.
         */
        cpu_lwp_free(l, 0);

        if (current) {
                /* Switch away into oblivion. */
                lwp_lock(l);
                spc_lock(l->l_cpu);
                mi_switch(l);
                panic("lwp_exit");
        }
}

/*
 * Free a dead LWP's remaining resources.
 *
 * XXXLWP limits.
 */
void
lwp_free(struct lwp *l, bool recycle, bool last)
{
        struct proc *p = l->l_proc;
        struct rusage *ru;
        ksiginfoq_t kq;

        KASSERT(l != curlwp);
        KASSERT(last || mutex_owned(p->p_lock));

        /*
         * We use the process credentials instead of the lwp credentials here
         * because the lwp credentials maybe cached (just after a setuid call)
         * and we don't want pay for syncing, since the lwp is going away
         * anyway
         */
        if (p != &proc0 && p->p_nlwps != 1)
                (void)chglwpcnt(kauth_cred_getuid(p->p_cred), -1);

        /*
         * In the unlikely event that the LWP is still on the CPU,
         * then spin until it has switched away.
         *
         * atomic_load_acquire matches atomic_store_release in
         * lwp_startup and mi_switch.
         */
        while (__predict_false((atomic_load_acquire(&l->l_pflag) & LP_RUNNING)
                != 0)) {
                SPINLOCK_BACKOFF_HOOK;
        }

        /*
         * Now that the LWP's known off the CPU, reset its state back to
         * LSIDL, which defeats anything that might have gotten a hold on
         * the LWP via pid_table before the ID was freed.  It's important
         * to do this with both the LWP locked and p_lock held.
         *
         * Also reset the CPU and lock pointer back to curcpu(), since the
         * LWP will in all likelyhood be cached with the current CPU in
         * lwp_cache when we free it and later allocated from there again
         * (avoid incidental lock contention).
         */
        lwp_lock(l);
        l->l_stat = LSIDL;
        l->l_cpu = curcpu();
        lwp_unlock_to(l, l->l_cpu->ci_schedstate.spc_lwplock);

        /*
         * If this was not the last LWP in the process, then adjust counters
         * and unlock.  This is done differently for the last LWP in exit1().
         */
        if (!last) {
                /*
                 * Add the LWP's run time to the process' base value.
                 * This needs to co-incide with coming off p_lwps.
                 */
                bintime_add(&p->p_rtime, &l->l_rtime);
                p->p_pctcpu += l->l_pctcpu;
                ru = &p->p_stats->p_ru;
                ruadd(ru, &l->l_ru);
                LIST_REMOVE(l, l_sibling);
                p->p_nlwps--;
                p->p_nzlwps--;
                if ((l->l_prflag & LPR_DETACHED) != 0)
                        p->p_ndlwps--;
                mutex_exit(p->p_lock);

                /*
                 * Have any LWPs sleeping in lwp_wait() recheck for
                 * deadlock.
                 */
                cv_broadcast(&p->p_lwpcv);

                /* Free the LWP ID. */
                mutex_enter(&proc_lock);
                proc_free_lwpid(p, l->l_lid);
                mutex_exit(&proc_lock);
        }

        /*
         * Destroy the LWP's remaining signal information.
         */
        ksiginfo_queue_init(&kq);
        sigclear(&l->l_sigpend, NULL, &kq);
        ksiginfo_queue_drain(&kq);
        cv_destroy(&l->l_sigcv);
        cv_destroy(&l->l_waitcv);

        /*
         * Free lwpctl structure and affinity.
         */
        if (l->l_lwpctl) {
                lwp_ctl_free(l);
        }
        if (l->l_affinity) {
                kcpuset_unuse(l->l_affinity, NULL);
                l->l_affinity = NULL;
        }

        /*
         * Free remaining data structures and the LWP itself unless the
         * caller wants to recycle.
         */
        if (l->l_name != NULL)
                kmem_free(l->l_name, MAXCOMLEN);

        kmsan_lwp_free(l);
        kcov_lwp_free(l);
        cpu_lwp_free2(l);
        uvm_lwp_exit(l);

        KASSERT(SLIST_EMPTY(&l->l_pi_lenders));
        KASSERT(l->l_inheritedprio == -1);
        KASSERT(l->l_blcnt == 0);
        kdtrace_thread_dtor(NULL, l);
        if (!recycle)
                pool_cache_put(lwp_cache, l);
}

/*
 * Migrate the LWP to the another CPU.  Unlocks the LWP.
 */
void
lwp_migrate(lwp_t *l, struct cpu_info *tci)
{
        struct schedstate_percpu *tspc;
        int lstat = l->l_stat;

        KASSERT(lwp_locked(l, NULL));
        KASSERT(tci != NULL);

        /* If LWP is still on the CPU, it must be handled like LSONPROC */
        if ((l->l_pflag & LP_RUNNING) != 0) {
                lstat = LSONPROC;
        }

        /*
         * The destination CPU could be changed while previous migration
         * was not finished.
         */
        if (l->l_target_cpu != NULL) {
                l->l_target_cpu = tci;
                lwp_unlock(l);
                return;
        }

        /* Nothing to do if trying to migrate to the same CPU */
        if (l->l_cpu == tci) {
                lwp_unlock(l);
                return;
        }

        KASSERT(l->l_target_cpu == NULL);
        tspc = &tci->ci_schedstate;
        switch (lstat) {
        case LSRUN:
                l->l_target_cpu = tci;
                break;
        case LSSLEEP:
                l->l_cpu = tci;
                break;
        case LSIDL:
        case LSSTOP:
        case LSSUSPENDED:
                l->l_cpu = tci;
                if (l->l_wchan == NULL) {
                        lwp_unlock_to(l, tspc->spc_lwplock);
                        return;
                }
                break;
        case LSONPROC:
                l->l_target_cpu = tci;
                spc_lock(l->l_cpu);
                sched_resched_cpu(l->l_cpu, PRI_USER_RT, true);
                /* spc now unlocked */
                break;
        }
        lwp_unlock(l);
}

#define lwp_find_exclude(l)                                     \
        ((l)->l_stat == LSIDL || (l)->l_stat == LSZOMB)

/*
 * Find the LWP in the process.  Arguments may be zero, in such case,
 * the calling process and first LWP in the list will be used.
 * On success - returns proc locked.
 *
 * => pid == 0 -> look in curproc.
 * => pid == -1 -> match any proc.
 * => otherwise look up the proc.
 *
 * => lid == 0 -> first LWP in the proc
 * => otherwise specific LWP
 */
struct lwp *
lwp_find2(pid_t pid, lwpid_t lid)
{
        proc_t *p;
        lwp_t *l;

        /* First LWP of specified proc. */
        if (lid == 0) {
                switch (pid) {
                case -1:
                        /* No lookup keys. */
                        return NULL;
                case 0:
                        p = curproc;
                        mutex_enter(p->p_lock);
                        break;
                default:
                        mutex_enter(&proc_lock);
                        p = proc_find(pid);
                        if (__predict_false(p == NULL)) {
                                mutex_exit(&proc_lock);
                                return NULL;
                        }
                        mutex_enter(p->p_lock);
                        mutex_exit(&proc_lock);
                        break;
                }
                LIST_FOREACH(l, &p->p_lwps, l_sibling) {
                        if (__predict_true(!lwp_find_exclude(l)))
                                break;
                }
                goto out;
        }

        l = proc_find_lwp_acquire_proc(lid, &p);
        if (l == NULL)
                return NULL;
        KASSERT(p != NULL);
        KASSERT(mutex_owned(p->p_lock));

        if (__predict_false(lwp_find_exclude(l))) {
                l = NULL;
                goto out;
        }

        /* Apply proc filter, if applicable. */
        switch (pid) {
        case -1:
                /* Match anything. */
                break;
        case 0:
                if (p != curproc)
                        l = NULL;
                break;
        default:
                if (p->p_pid != pid)
                        l = NULL;
                break;
        }

 out:
        if (__predict_false(l == NULL)) {
                mutex_exit(p->p_lock);
        }
        return l;
}

/*
 * Look up a live LWP within the specified process.
 *
 * Must be called with p->p_lock held (as it looks at the radix tree,
 * and also wants to exclude idle and zombie LWPs).
 */
struct lwp *
lwp_find(struct proc *p, lwpid_t id)
{
        struct lwp *l;

        KASSERT(mutex_owned(p->p_lock));

        l = proc_find_lwp(p, id);
        KASSERT(l == NULL || l->l_lid == id);

        /*
         * No need to lock - all of these conditions will
         * be visible with the process level mutex held.
         */
        if (__predict_false(l != NULL && lwp_find_exclude(l)))
                l = NULL;

        return l;
}

/*
 * Verify that an LWP is locked, and optionally verify that the lock matches
 * one we specify.
 */
int
lwp_locked(struct lwp *l, kmutex_t *mtx)
{
        kmutex_t *cur = l->l_mutex;

        return mutex_owned(cur) && (mtx == cur || mtx == NULL);
}

/*
 * Lend a new mutex to an LWP.  The old mutex must be held.
 */
kmutex_t *
lwp_setlock(struct lwp *l, kmutex_t *mtx)
{
        kmutex_t *oldmtx = l->l_mutex;

        KASSERT(mutex_owned(oldmtx));

        atomic_store_release(&l->l_mutex, mtx);
        return oldmtx;
}

/*
 * Lend a new mutex to an LWP, and release the old mutex.  The old mutex
 * must be held.
 */
void
lwp_unlock_to(struct lwp *l, kmutex_t *mtx)
{
        kmutex_t *old;

        KASSERT(lwp_locked(l, NULL));

        old = l->l_mutex;
        atomic_store_release(&l->l_mutex, mtx);
        mutex_spin_exit(old);
}

int
lwp_trylock(struct lwp *l)
{
        kmutex_t *old;

        for (;;) {
                if (!mutex_tryenter(old = atomic_load_consume(&l->l_mutex)))
                        return 0;
                if (__predict_true(atomic_load_relaxed(&l->l_mutex) == old))
                        return 1;
                mutex_spin_exit(old);
        }
}

void
lwp_unsleep(lwp_t *l, bool unlock)
{

        KASSERT(mutex_owned(l->l_mutex));
        (*l->l_syncobj->sobj_unsleep)(l, unlock);
}

/*
 * Lock an LWP.
 */
void
lwp_lock(lwp_t *l)
{
        kmutex_t *old = atomic_load_consume(&l->l_mutex);

        /*
         * Note: mutex_spin_enter() will have posted a read barrier.
         * Re-test l->l_mutex.  If it has changed, we need to try again.
         */
        mutex_spin_enter(old);
        while (__predict_false(atomic_load_relaxed(&l->l_mutex) != old)) {
                mutex_spin_exit(old);
                old = atomic_load_consume(&l->l_mutex);
                mutex_spin_enter(old);
        }
}

/*
 * Unlock an LWP.
 */
void
lwp_unlock(lwp_t *l)
{

        mutex_spin_exit(l->l_mutex);
}

void
lwp_changepri(lwp_t *l, pri_t pri)
{

        KASSERT(mutex_owned(l->l_mutex));

        if (l->l_priority == pri)
                return;

        (*l->l_syncobj->sobj_changepri)(l, pri);
        KASSERT(l->l_priority == pri);
}

void
lwp_lendpri(lwp_t *l, pri_t pri)
{
        KASSERT(mutex_owned(l->l_mutex));

        (*l->l_syncobj->sobj_lendpri)(l, pri);
        KASSERT(l->l_inheritedprio == pri);
}

pri_t
lwp_eprio(lwp_t *l)
{
        pri_t pri = l->l_priority;

        KASSERT(mutex_owned(l->l_mutex));

        /*
         * Timeshared/user LWPs get a temporary priority boost for blocking
         * in kernel.  This is key to good interactive response on a loaded
         * system: without it, things will seem very sluggish to the user. 
         *
         * The function of the boost is to get the LWP onto a CPU and
         * running quickly.  Once that happens the LWP loses the priority
         * boost and could be preempted very quickly by another LWP but that
         * won't happen often enough to be an annoyance.
         */
        if (pri <= MAXPRI_USER && l->l_boostpri > MAXPRI_USER)
                pri = (pri >> 1) + l->l_boostpri;

        return MAX(l->l_auxprio, pri);
}

/*
 * Handle exceptions for mi_userret().  Called if a member of LW_USERRET is
 * set or a preemption is required.
 */
void
lwp_userret(struct lwp *l)
{
        struct proc *p;
        int sig, f;

        KASSERT(l == curlwp);
        KASSERT(l->l_stat == LSONPROC);
        p = l->l_proc;

        for (;;) {
                /*
                 * This is the main location that user preemptions are
                 * processed.
                 */
                preempt_point();

                /*
                 * It is safe to do this unlocked and without raised SPL,
                 * since whenever a flag of interest is added to l_flag the
                 * LWP will take an AST and come down this path again.  If a
                 * remote CPU posts the AST, it will be done with an IPI
                 * (strongly synchronising).
                 */
                if ((f = atomic_load_relaxed(&l->l_flag) & LW_USERRET) == 0) {
                        return;
                }

                /*
                 * Start out with the correct credentials.
                 */
                if ((f & LW_CACHECRED) != 0) {
                        kauth_cred_t oc = l->l_cred;
                        mutex_enter(p->p_lock);
                        l->l_cred = kauth_cred_hold(p->p_cred);
                        lwp_lock(l);
                        l->l_flag &= ~LW_CACHECRED;
                        lwp_unlock(l);
                        mutex_exit(p->p_lock);
                        kauth_cred_free(oc);
                }

                /*
                 * Process pending signals first, unless the process
                 * is dumping core or exiting, where we will instead
                 * enter the LW_WSUSPEND case below.
                 */
                if ((f & (LW_PENDSIG | LW_WCORE | LW_WEXIT)) == LW_PENDSIG) {
                        mutex_enter(p->p_lock);
                        while ((sig = issignal(l)) != 0)
                                postsig(sig);
                        mutex_exit(p->p_lock);
                        continue;
                }

                /*
                 * Core-dump or suspend pending.
                 *
                 * In case of core dump, suspend ourselves, so that the kernel
                 * stack and therefore the userland registers saved in the
                 * trapframe are around for coredump() to write them out.
                 * We also need to save any PCU resources that we have so that
                 * they accessible for coredump().  We issue a wakeup on
                 * p->p_lwpcv so that sigexit() will write the core file out
                 * once all other LWPs are suspended.  
                 */
                if ((f & LW_WSUSPEND) != 0) {
                        pcu_save_all(l);
                        mutex_enter(p->p_lock);
                        p->p_nrlwps--;
                        lwp_lock(l);
                        l->l_stat = LSSUSPENDED;
                        lwp_unlock(l);
                        mutex_exit(p->p_lock);
                        cv_broadcast(&p->p_lwpcv);
                        lwp_lock(l);
                        spc_lock(l->l_cpu);
                        mi_switch(l);
                        continue;
                }

                /*
                 * Process is exiting.  The core dump and signal cases must
                 * be handled first.
                 */
                if ((f & LW_WEXIT) != 0) {
                        lwp_exit(l);
                        KASSERT(0);
                        /* NOTREACHED */
                }

                /*
                 * Update lwpctl processor (for vfork child_return).
                 */
                if ((f & LW_LWPCTL) != 0) {
                        lwp_lock(l);
                        KASSERT(kpreempt_disabled());
                        l->l_lwpctl->lc_curcpu = (int)cpu_index(l->l_cpu);
                        l->l_lwpctl->lc_pctr++;
                        l->l_flag &= ~LW_LWPCTL;
                        lwp_unlock(l);
                        continue;
                }
        }
}

/*
 * Force an LWP to enter the kernel, to take a trip through lwp_userret().
 */
void
lwp_need_userret(struct lwp *l)
{

        KASSERT(!cpu_intr_p());
        KASSERT(lwp_locked(l, NULL) || l->l_stat == LSIDL);

        /*
         * If the LWP is in any state other than LSONPROC, we know that it
         * is executing in-kernel and will hit userret() on the way out. 
         *
         * If the LWP is curlwp, then we know we'll be back out to userspace
         * soon (can't be called from a hardware interrupt here).
         *
         * Otherwise, we can't be sure what the LWP is doing, so first make
         * sure the update to l_flag will be globally visible, and then
         * force the LWP to take a trip through trap() where it will do
         * userret().
         */
        if (l->l_stat == LSONPROC && l != curlwp) {
                membar_producer();
                cpu_signotify(l);
        }
}

/*
 * Add one reference to an LWP.  This will prevent the LWP from
 * exiting, thus keep the lwp structure and PCB around to inspect.
 */
void
lwp_addref(struct lwp *l)
{
        KASSERT(mutex_owned(l->l_proc->p_lock));
        KASSERT(l->l_stat != LSZOMB);
        l->l_refcnt++;
}

/*
 * Remove one reference to an LWP.  If this is the last reference,
 * then we must finalize the LWP's death.
 */
void
lwp_delref(struct lwp *l)
{
        struct proc *p = l->l_proc;

        mutex_enter(p->p_lock);
        lwp_delref2(l);
        mutex_exit(p->p_lock);
}

/*
 * Remove one reference to an LWP.  If this is the last reference,
 * then we must finalize the LWP's death.  The proc mutex is held
 * on entry.
 */
void
lwp_delref2(struct lwp *l)
{
        struct proc *p = l->l_proc;

        KASSERT(mutex_owned(p->p_lock));
        KASSERT(l->l_stat != LSZOMB);
        KASSERT(l->l_refcnt > 0);

        if (--l->l_refcnt == 0)
                cv_broadcast(&p->p_lwpcv);
}

/*
 * Drain all references to the current LWP.  Returns true if
 * we blocked.
 */
bool
lwp_drainrefs(struct lwp *l)
{
        struct proc *p = l->l_proc;
        bool rv = false;

        KASSERT(mutex_owned(p->p_lock));

        l->l_prflag |= LPR_DRAINING;

        while (l->l_refcnt > 0) {
                rv = true;
                cv_wait(&p->p_lwpcv, p->p_lock);
        }
        return rv;
}

/*
 * Return true if the specified LWP is 'alive'.  Only p->p_lock need
 * be held.
 */
bool
lwp_alive(lwp_t *l)
{

        KASSERT(mutex_owned(l->l_proc->p_lock));

        switch (l->l_stat) {
        case LSSLEEP:
        case LSRUN:
        case LSONPROC:
        case LSSTOP:
        case LSSUSPENDED:
                return true;
        default:
                return false;
        }
}

/*
 * Return first live LWP in the process.
 */
lwp_t *
lwp_find_first(proc_t *p)
{
        lwp_t *l;

        KASSERT(mutex_owned(p->p_lock));

        LIST_FOREACH(l, &p->p_lwps, l_sibling) {
                if (lwp_alive(l)) {
                        return l;
                }
        }

        return NULL;
}

/*
 * Allocate a new lwpctl structure for a user LWP.
 */
int
lwp_ctl_alloc(vaddr_t *uaddr)
{
        lcproc_t *lp;
        u_int bit, i, offset;
        struct uvm_object *uao;
        int error;
        lcpage_t *lcp;
        proc_t *p;
        lwp_t *l;

        l = curlwp;
        p = l->l_proc;

        /* don't allow a vforked process to create lwp ctls */
        if (p->p_lflag & PL_PPWAIT)
                return SET_ERROR(EBUSY);

        if (l->l_lcpage != NULL) {
                lcp = l->l_lcpage;
                *uaddr = lcp->lcp_uaddr + (vaddr_t)l->l_lwpctl - lcp->lcp_kaddr;
                return 0;
        }

        /* First time around, allocate header structure for the process. */
        if ((lp = p->p_lwpctl) == NULL) {
                lp = kmem_alloc(sizeof(*lp), KM_SLEEP);
                mutex_init(&lp->lp_lock, MUTEX_DEFAULT, IPL_NONE);
                lp->lp_uao = NULL;
                TAILQ_INIT(&lp->lp_pages);
                mutex_enter(p->p_lock);
                if (p->p_lwpctl == NULL) {
                        p->p_lwpctl = lp;
                        mutex_exit(p->p_lock);
                } else {
                        mutex_exit(p->p_lock);
                        mutex_destroy(&lp->lp_lock);
                        kmem_free(lp, sizeof(*lp));
                        lp = p->p_lwpctl;
                }
        }

        /*
         * Set up an anonymous memory region to hold the shared pages.
         * Map them into the process' address space.  The user vmspace
         * gets the first reference on the UAO.
         */
        mutex_enter(&lp->lp_lock);
        if (lp->lp_uao == NULL) {
                lp->lp_uao = uao_create(LWPCTL_UAREA_SZ, 0);
                lp->lp_cur = 0;
                lp->lp_max = LWPCTL_UAREA_SZ;
                lp->lp_uva = p->p_emul->e_vm_default_addr(p,
                     (vaddr_t)p->p_vmspace->vm_daddr, LWPCTL_UAREA_SZ,
                     p->p_vmspace->vm_map.flags & VM_MAP_TOPDOWN);
                error = uvm_map(&p->p_vmspace->vm_map, &lp->lp_uva,
                    LWPCTL_UAREA_SZ, lp->lp_uao, 0, 0, UVM_MAPFLAG(UVM_PROT_RW,
                    UVM_PROT_RW, UVM_INH_NONE, UVM_ADV_NORMAL, 0));
                if (error != 0) {
                        uao_detach(lp->lp_uao);
                        lp->lp_uao = NULL;
                        mutex_exit(&lp->lp_lock);
                        return error;
                }
        }

        /* Get a free block and allocate for this LWP. */
        TAILQ_FOREACH(lcp, &lp->lp_pages, lcp_chain) {
                if (lcp->lcp_nfree != 0)
                        break;
        }
        if (lcp == NULL) {
                /* Nothing available - try to set up a free page. */
                if (lp->lp_cur == lp->lp_max) {
                        mutex_exit(&lp->lp_lock);
                        return SET_ERROR(ENOMEM);
                }
                lcp = kmem_alloc(LWPCTL_LCPAGE_SZ, KM_SLEEP);

                /*
                 * Wire the next page down in kernel space.  Since this
                 * is a new mapping, we must add a reference.
                 */
                uao = lp->lp_uao;
                (*uao->pgops->pgo_reference)(uao);
                lcp->lcp_kaddr = vm_map_min(kernel_map);
                error = uvm_map(kernel_map, &lcp->lcp_kaddr, PAGE_SIZE,
                    uao, lp->lp_cur, PAGE_SIZE,
                    UVM_MAPFLAG(UVM_PROT_RW, UVM_PROT_RW,
                    UVM_INH_NONE, UVM_ADV_RANDOM, 0));
                if (error != 0) {
                        mutex_exit(&lp->lp_lock);
                        kmem_free(lcp, LWPCTL_LCPAGE_SZ);
                        (*uao->pgops->pgo_detach)(uao);
                        return error;
                }
                error = uvm_map_pageable(kernel_map, lcp->lcp_kaddr,
                    lcp->lcp_kaddr + PAGE_SIZE, FALSE, 0);
                if (error != 0) {
                        mutex_exit(&lp->lp_lock);
                        uvm_unmap(kernel_map, lcp->lcp_kaddr,
                            lcp->lcp_kaddr + PAGE_SIZE);
                        kmem_free(lcp, LWPCTL_LCPAGE_SZ);
                        return error;
                }
                /* Prepare the page descriptor and link into the list. */
                lcp->lcp_uaddr = lp->lp_uva + lp->lp_cur;
                lp->lp_cur += PAGE_SIZE;
                lcp->lcp_nfree = LWPCTL_PER_PAGE;
                lcp->lcp_rotor = 0;
                memset(lcp->lcp_bitmap, 0xff, LWPCTL_BITMAP_SZ);
                TAILQ_INSERT_HEAD(&lp->lp_pages, lcp, lcp_chain);
        }
        for (i = lcp->lcp_rotor; lcp->lcp_bitmap[i] == 0;) {
                if (++i >= LWPCTL_BITMAP_ENTRIES)
                        i = 0;
        }
        bit = ffs(lcp->lcp_bitmap[i]) - 1;
        lcp->lcp_bitmap[i] ^= (1U << bit);
        lcp->lcp_rotor = i;
        lcp->lcp_nfree--;
        l->l_lcpage = lcp;
        offset = (i << 5) + bit;
        l->l_lwpctl = (lwpctl_t *)lcp->lcp_kaddr + offset;
        *uaddr = lcp->lcp_uaddr + offset * sizeof(lwpctl_t);
        mutex_exit(&lp->lp_lock);

        KPREEMPT_DISABLE(l);
        l->l_lwpctl->lc_curcpu = (int)cpu_index(curcpu());
        KPREEMPT_ENABLE(l);

        return 0;
}

/*
 * Free an lwpctl structure back to the per-process list.
 */
void
lwp_ctl_free(lwp_t *l)
{
        struct proc *p = l->l_proc;
        lcproc_t *lp;
        lcpage_t *lcp;
        u_int map, offset;

        /* don't free a lwp context we borrowed for vfork */
        if (p->p_lflag & PL_PPWAIT) {
                l->l_lwpctl = NULL;
                return;
        }

        lp = p->p_lwpctl;
        KASSERT(lp != NULL);

        lcp = l->l_lcpage;
        offset = (u_int)((lwpctl_t *)l->l_lwpctl - (lwpctl_t *)lcp->lcp_kaddr);
        KASSERT(offset < LWPCTL_PER_PAGE);

        mutex_enter(&lp->lp_lock);
        lcp->lcp_nfree++;
        map = offset >> 5;
        lcp->lcp_bitmap[map] |= (1U << (offset & 31));
        if (lcp->lcp_bitmap[lcp->lcp_rotor] == 0)
                lcp->lcp_rotor = map;
        if (TAILQ_FIRST(&lp->lp_pages)->lcp_nfree == 0) {
                TAILQ_REMOVE(&lp->lp_pages, lcp, lcp_chain);
                TAILQ_INSERT_HEAD(&lp->lp_pages, lcp, lcp_chain);
        }
        mutex_exit(&lp->lp_lock);
}

/*
 * Process is exiting; tear down lwpctl state.  This can only be safely
 * called by the last LWP in the process.
 */
void
lwp_ctl_exit(void)
{
        lcpage_t *lcp, *next;
        lcproc_t *lp;
        proc_t *p;
        lwp_t *l;

        l = curlwp;
        l->l_lwpctl = NULL;
        l->l_lcpage = NULL;
        p = l->l_proc;
        lp = p->p_lwpctl;

        KASSERT(lp != NULL);
        KASSERT(p->p_nlwps == 1);

        for (lcp = TAILQ_FIRST(&lp->lp_pages); lcp != NULL; lcp = next) {
                next = TAILQ_NEXT(lcp, lcp_chain);
                uvm_unmap(kernel_map, lcp->lcp_kaddr,
                    lcp->lcp_kaddr + PAGE_SIZE);
                kmem_free(lcp, LWPCTL_LCPAGE_SZ);
        }

        if (lp->lp_uao != NULL) {
                uvm_unmap(&p->p_vmspace->vm_map, lp->lp_uva,
                    lp->lp_uva + LWPCTL_UAREA_SZ);
        }

        mutex_destroy(&lp->lp_lock);
        kmem_free(lp, sizeof(*lp));
        p->p_lwpctl = NULL;
}

/*
 * Return the current LWP's "preemption counter".  Used to detect
 * preemption across operations that can tolerate preemption without
 * crashing, but which may generate incorrect results if preempted.
 *
 * We do arithmetic in unsigned long to avoid undefined behaviour in
 * the event of arithmetic overflow on LP32, and issue __insn_barrier()
 * on both sides so this can safely be used to detect changes to the
 * preemption counter in loops around other memory accesses even in the
 * event of whole-program optimization (e.g., gcc -flto).
 */
long
lwp_pctr(void)
{
        unsigned long pctr;

        __insn_barrier();
        pctr = curlwp->l_ru.ru_nvcsw;
        pctr += curlwp->l_ru.ru_nivcsw;
        __insn_barrier();
        return pctr;
}

/*
 * Set an LWP's private data pointer.
 */
int
lwp_setprivate(struct lwp *l, void *ptr)
{
        int error = 0;

        l->l_private = ptr;
#ifdef __HAVE_CPU_LWP_SETPRIVATE
        error = cpu_lwp_setprivate(l, ptr);
#endif
        return error;
}

/*
 * Perform any thread-related cleanup on LWP exit.
 * N.B. l->l_proc->p_lock must be HELD on entry but will
 * be released before returning!
 */
void
lwp_thread_cleanup(struct lwp *l)
{

        KASSERT(mutex_owned(l->l_proc->p_lock));
        mutex_exit(l->l_proc->p_lock);

        /*
         * If the LWP has robust futexes, release them all
         * now.
         */
        if (__predict_false(l->l_robust_head != 0)) {
                futex_release_all_lwp(l);
        }
}

#if defined(DDB)
#include <machine/pcb.h>

void
lwp_whatis(uintptr_t addr, void (*pr)(const char *, ...))
{
        lwp_t *l;

        LIST_FOREACH(l, &alllwp, l_list) {
                uintptr_t stack = (uintptr_t)KSTACK_LOWEST_ADDR(l);

                if (addr < stack || stack + KSTACK_SIZE <= addr) {
                        continue;
                }
                (*pr)("%p is %p+%zu, LWP %p's stack\n",
                    (void *)addr, (void *)stack,
                    (size_t)(addr - stack), l);
        }
}
#endif /* defined(DDB) */