/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License, Version 1.0 only
 * (the "License").  You may not use this file except in compliance
 * with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or http://www.opensolaris.org/os/licensing.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */
/*
 * Copyright 2004 Sun Microsystems, Inc.  All rights reserved.
 * Use is subject to license terms.
 */

/*
 * Kernel protection serializers: general purpose synchronization mechanism.
 *
 * Serializers provide a simple way to serialize access to some resource. They
 * can be used as an alternative to locks or STREAMS perimeters. They scale
 * much better than STREAMS outer serializers.
 *
 * Serializer is an abstraction that guarantees that all functions executed
 * within the serializer are serialized: they are executed in the order they
 * entered serializer one at a time.
 *
 * INTERFACES:
 *
 * serializer_t *serializer_create(flags);
 *
 *      Create a serializer. The flags may be either SER_SLEEP or SER_NOSLEEP
 *      which are the same as KM_SLEEP and KM_NOSLEEP respectively.
 *
 * serializer_enter(serializer, proc, mblk, arg);
 *
 *      Execute 'proc(mblk, arg)' within the serializer.
 *
 * serializer_wait(serializer);
 *
 *      Wait for pending serializer jobs to complete. This function should never
 *      be called within the serializer or it will deadlock.
 *
 * serializer_destroy(serializer);
 *
 *      Destroy serializer.
 *
 * Serializers export three DTrace SDT probes:
 *
 *      serializer-enqueue(serializer, mblk, arg, proc)
 *
 *              The probe triggers when serializer is busy and the request is
 *              queued.
 *
 *      serializer-exec-start(serializer, mblk, arg, proc)
 *
 *              The probe triggers before the request is executed
 *
 *      serializer-exec-end(serializer, mblk, arg, proc)
 *
 *              The probe triggers after the request is executed
 *
 *
 * IMPLEMENTATION.
 *
 * Serializer consists of a "owner" and a list of queued jobs. The first thread
 * entering serializer sets the owner and executes its job directly without
 * context switch. Then it processes jobs which may have been enqueued while it
 * was executing a job and drops the owner, leaving the serializer empty.  Any
 * thread entering an owned serializer enqueues its job and returns immediately.
 *
 * Serializer data structure holds several fields used for debugging only. They
 * are not relevant for the proper serializer functioning.
 *
 * When new requests arrive faster then they are processed it is possible that a
 * thread that started processing serializer will continue doing so for a long
 * time. To avoid such pathological behavior the amount of requests drained by
 * serializer_enter() is limited by `serializer_credit' value. After the credit
 * is expired serializer_enter() schedules a taskq request to continue draining.
 * The taskq thread draining is not limited by serializer_credit. Note that it
 * is possible that another serializer_enter() will drain the serializer before
 * a taskq thread will get to it.
 */

#include <sys/types.h>
#include <sys/kmem.h>
#include <sys/thread.h>
#include <sys/mutex.h>
#include <sys/systm.h>
#include <sys/debug.h>
#include <sys/taskq.h>
#include <sys/sdt.h>
#include <sys/serializer.h>

#define SERIALIZER_NAMELEN 31

/*
 * Serializer abstraction.
 * Fields marked (D) are used for debugging purposes only.
 */
struct serializer_s {
        kmutex_t        ser_lock;       /* Protects state and the list */
        kthread_t       *ser_owner;     /* Thread executing serializer */
        ushort_t        ser_taskq;      /* Serializer scheduled for taskq */
        kcondvar_t      ser_cv;         /* For serializer-wait */
        uint_t          ser_count;      /* # of queued requests (D) */
        mblk_t          *ser_first;     /* First message in the queue */
        mblk_t          *ser_last;      /* Last message in the queue */
        srproc_t        *ser_proc;      /* Currently executing proc (D) */
        mblk_t          *ser_curr;      /* Currently executing msg (D) */
        void            *ser_arg;       /* Currently executing arg (D) */
};

static kmem_cache_t *serializer_cache;

/*
 * How many drains are allowed before we switch to taskq processing.
 */
#define SERIALIZER_CREDIT 10
static int serializer_credit = SERIALIZER_CREDIT;

/* Statistics for debugging */
static int perim_context_swtch = 0;

static int serializer_constructor(void *, void *, int);
static void serializer_destructor(void *, void *);
static void serializer_exec(serializer_t *, srproc_t, mblk_t *, void *);
static void serializer_enqueue(serializer_t *, srproc_t, mblk_t *, void *);
static void serializer_drain(serializer_t *, int);
static void serializer_drain_completely(serializer_t *);

/*
 * SERIALIZER Implementation.
 */

/*
 * Record debugging information and execute single request.
 */
static void
serializer_exec(serializer_t *s, srproc_t proc, mblk_t *mp, void *arg)
{
        ASSERT(MUTEX_NOT_HELD(&s->ser_lock));
        ASSERT(s->ser_owner == curthread);

        ASSERT(proc != NULL);
        ASSERT(mp != NULL);

        s->ser_curr = mp;
        s->ser_arg = arg;
        s->ser_proc = proc;
        proc(mp, arg);
}

/*
 * Enqueue a single request on serializer.
 */
static void
serializer_enqueue(serializer_t *s, srproc_t proc, mblk_t *mp, void *arg)
{
        ASSERT(MUTEX_HELD(&s->ser_lock));

        DTRACE_PROBE4(serializer__enqueue, serializer_t *, s,
            mblk_t *, mp, void *, arg, srproc_t, proc);
        s->ser_count++;
        mp->b_queue = (queue_t *)proc;
        mp->b_prev = (mblk_t *)arg;
        if (s->ser_last != NULL)
                s->ser_last->b_next = mp;
        else
                s->ser_first = mp;
        s->ser_last = mp;
}

/*
 * Drain serializer, limiting drain to `credit' requests at most.
 */
static void
serializer_drain(serializer_t *s, int credit)
{
        mblk_t *mp = s->ser_first;

        ASSERT(MUTEX_HELD(&s->ser_lock));
        ASSERT(s->ser_owner == curthread);

        for (; mp != NULL && credit-- != 0; mp = s->ser_first) {
                srproc_t *proc = (srproc_t *)mp->b_queue;
                void *arg = mp->b_prev;

                if ((s->ser_first = s->ser_first->b_next) == NULL) {
                        s->ser_last = NULL;
                } else {
                        mp->b_next = NULL;
                }
                ASSERT(s->ser_count != 0);
                s->ser_count--;
                mp->b_queue = NULL;
                mp->b_prev = NULL;
                mutex_exit(&s->ser_lock);

                DTRACE_PROBE4(serializer__exec__start, serializer_t *, s,
                    mblk_t *, mp, void *, arg, srproc_t, proc);
                serializer_exec(s, proc, mp, arg);
                DTRACE_PROBE4(serializer__exec__end, serializer_t *, s,
                    mblk_t *, mp, void *, arg, srproc_t, proc);

                mutex_enter(&s->ser_lock);
        }
}

/*
 * Drain serializer completely if serializer is free.
 */
static void
serializer_drain_completely(serializer_t *s)
{
        mutex_enter(&s->ser_lock);
        ASSERT(s->ser_taskq);
        if (s->ser_owner == NULL) {
                s->ser_owner = curthread;
                while (s->ser_first != NULL)
                        serializer_drain(s, INT_MAX);
                s->ser_owner = NULL;
                s->ser_curr = NULL;
                s->ser_proc = NULL;
                s->ser_arg = NULL;
        }
        s->ser_taskq = B_FALSE;
        /*
         * Wake up serializer_wait().
         */
        cv_signal(&s->ser_cv);
        mutex_exit(&s->ser_lock);
}

/*
 * Call proc(mp, arg) within serializer.
 *
 * If serializer is empty and not owned, proc(mp, arg) is called right
 * away. Otherwise the request is queued.
 */
void
serializer_enter(serializer_t *s, srproc_t proc, mblk_t *mp, void *arg)
{
        ASSERT(proc != NULL);
        ASSERT(mp != NULL);
        ASSERT(mp->b_next == NULL);
        ASSERT(mp->b_prev == NULL);

        ASSERT(MUTEX_NOT_HELD(&s->ser_lock));

        mutex_enter(&s->ser_lock);
        if (s->ser_owner != NULL) {
                /*
                 * Serializer is owned. Enqueue and return.
                 */
                serializer_enqueue(s, proc, mp, arg);
        } else {
                taskqid_t tid = TASKQID_INVALID;

                /*
                 * If the request list is empty, can process right away,
                 * otherwise enqueue and process.
                 */
                s->ser_owner = curthread;

                if (s->ser_first != NULL) {
                        ASSERT(s->ser_count != 0);
                        serializer_enqueue(s, proc, mp, arg);
                } else {
                        ASSERT(s->ser_count == 0);
                        mutex_exit(&s->ser_lock);
                        /*
                         * Execute request
                         */
                        DTRACE_PROBE4(serializer__exec__start,
                            serializer_t *, s, mblk_t *, mp,
                            void *, arg, srproc_t, proc);
                        serializer_exec(s, proc, mp, arg);
                        DTRACE_PROBE4(serializer__exec__end,
                            serializer_t *, s, mblk_t *, mp,
                            void *, arg, srproc_t, proc);
                        mutex_enter(&s->ser_lock);
                }

                /*
                 * Drain whatever has arrived in the meantime.
                 * If we spend too much time draining, continue draining by the
                 * taskq thread.
                 */
                while ((s->ser_first != NULL) && (tid == 0)) {
                        serializer_drain(s, serializer_credit);
                        if (s->ser_first != NULL) {
                                perim_context_swtch++;
                                /*
                                 * If there is a taskq pending for this
                                 * serializer, no need to schedule a new one.
                                 */
                                if (s->ser_taskq) {
                                        break;
                                } else {
                                        tid = taskq_dispatch(system_taskq,
                                            (task_func_t *)
                                            serializer_drain_completely,
                                            s, TQ_NOSLEEP | TQ_NOQUEUE);
                                        if (tid != TASKQID_INVALID)
                                                s->ser_taskq = B_TRUE;
                                }
                        }
                }
                s->ser_owner = NULL;
                s->ser_curr = NULL;
                s->ser_proc = NULL;
                s->ser_arg = NULL;
        }
        /*
         * Wakeup serializer_wait().
         */
        cv_signal(&s->ser_cv);
        mutex_exit(&s->ser_lock);
}

/*
 * Wait for pending serializer jobs to complete. This function should never be
 * called within the serializer or it will deadlock.
 */
void
serializer_wait(serializer_t *s)
{
        mutex_enter(&s->ser_lock);

        ASSERT(s->ser_owner != curthread);

        while ((s->ser_owner != NULL) || s->ser_taskq || (s->ser_first != NULL))
                cv_wait(&s->ser_cv, &s->ser_lock);
        ASSERT((s->ser_first == NULL) && (s->ser_last == NULL));
        /*
         * Wakeup other potential waiters.
         */
        cv_signal(&s->ser_cv);
        mutex_exit(&s->ser_lock);
}

/*
 * Create a new serializer.
 */
serializer_t *
serializer_create(int flags)
{
        return (kmem_cache_alloc(serializer_cache, flags));
}

/*
 * Wait for all pending entries to drain and then destroy serializer.
 */
void
serializer_destroy(serializer_t *s)
{
        serializer_wait(s);

        ASSERT(s->ser_owner == NULL);
        ASSERT(s->ser_taskq == 0);
        ASSERT(s->ser_count == 0);
        ASSERT(s->ser_first == NULL);
        ASSERT(s->ser_last == NULL);

        kmem_cache_free(serializer_cache, s);
}

/*ARGSUSED*/
static int
serializer_constructor(void *buf, void *cdrarg, int kmflags)
{
        serializer_t *s = buf;

        mutex_init(&s->ser_lock, NULL, MUTEX_DEFAULT, NULL);
        cv_init(&s->ser_cv, NULL, CV_DEFAULT, NULL);

        s->ser_taskq = 0;
        s->ser_count = 0;
        s->ser_first = s->ser_last = s->ser_curr = NULL;
        s->ser_proc = NULL;
        s->ser_arg = NULL;
        s->ser_owner = NULL;
        return (0);
}

/*ARGSUSED*/
static void
serializer_destructor(void *buf, void *cdrarg)
{
        serializer_t *s = buf;

        mutex_destroy(&s->ser_lock);
        cv_destroy(&s->ser_cv);
}

void
serializer_init(void)
{
        serializer_cache = kmem_cache_create("serializer_cache",
            sizeof (serializer_t), 0, serializer_constructor,
            serializer_destructor, NULL, NULL, NULL, 0);
}