// SPDX-License-Identifier: GPL-2.0-only /* * Context tracking: Probe on high level context boundaries such as kernel, * userspace, guest or idle. * * This is used by RCU to remove its dependency on the timer tick while a CPU * runs in idle, userspace or guest mode. * * User/guest tracking started by Frederic Weisbecker: * * Copyright (C) 2012 Red Hat, Inc., Frederic Weisbecker * * Many thanks to Gilad Ben-Yossef, Paul McKenney, Ingo Molnar, Andrew Morton, * Steven Rostedt, Peter Zijlstra for suggestions and improvements. * * RCU extended quiescent state bits imported from kernel/rcu/tree.c * where the relevant authorship may be found. */ #include <linux/context_tracking.h> #include <linux/rcupdate.h> #include <linux/sched.h> #include <linux/hardirq.h> #include <linux/export.h> #include <linux/kprobes.h> #include <trace/events/rcu.h> DEFINE_PER_CPU(struct context_tracking, context_tracking) = { #ifdef CONFIG_CONTEXT_TRACKING_IDLE .nesting = 1, .nmi_nesting = CT_NESTING_IRQ_NONIDLE, #endif .state = ATOMIC_INIT(CT_RCU_WATCHING), }; EXPORT_SYMBOL_GPL(context_tracking); #ifdef CONFIG_CONTEXT_TRACKING_IDLE #define TPS(x) tracepoint_string(x) /* Record the current task on exiting RCU-tasks (dyntick-idle entry). */ static __always_inline void rcu_task_exit(void) { #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) WRITE_ONCE(current->rcu_tasks_idle_cpu, smp_processor_id()); #endif /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */ } /* Record no current task on entering RCU-tasks (dyntick-idle exit). */ static __always_inline void rcu_task_enter(void) { #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) WRITE_ONCE(current->rcu_tasks_idle_cpu, -1); #endif /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */ } /* * Record entry into an extended quiescent state. This is only to be * called when not already in an extended quiescent state, that is, * RCU is watching prior to the call to this function and is no longer * watching upon return. */ static noinstr void ct_kernel_exit_state(int offset) { /* * CPUs seeing atomic_add_return() must see prior RCU read-side * critical sections, and we also must force ordering with the * next idle sojourn. */ // RCU is still watching. Better not be in extended quiescent state! WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !rcu_is_watching_curr_cpu()); (void)ct_state_inc(offset); // RCU is no longer watching. } /* * Record exit from an extended quiescent state. This is only to be * called from an extended quiescent state, that is, RCU is not watching * prior to the call to this function and is watching upon return. */ static noinstr void ct_kernel_enter_state(int offset) { int seq; /* * CPUs seeing atomic_add_return() must see prior idle sojourns, * and we also must force ordering with the next RCU read-side * critical section. */ seq = ct_state_inc(offset); // RCU is now watching. Better not be in an extended quiescent state! WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !(seq & CT_RCU_WATCHING)); } /* * Enter an RCU extended quiescent state, which can be either the * idle loop or adaptive-tickless usermode execution. * * We crowbar the ->nmi_nesting field to zero to allow for * the possibility of usermode upcalls having messed up our count * of interrupt nesting level during the prior busy period. */ static void noinstr ct_kernel_exit(bool user, int offset) { struct context_tracking *ct = this_cpu_ptr(&context_tracking); WARN_ON_ONCE(ct_nmi_nesting() != CT_NESTING_IRQ_NONIDLE); WRITE_ONCE(ct->nmi_nesting, 0); WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && ct_nesting() == 0); if (ct_nesting() != 1) { // RCU will still be watching, so just do accounting and leave. ct->nesting--; return; } instrumentation_begin(); lockdep_assert_irqs_disabled(); trace_rcu_watching(TPS("End"), ct_nesting(), 0, ct_rcu_watching()); WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current)); rcu_preempt_deferred_qs(current); // instrumentation for the noinstr ct_kernel_exit_state() instrument_atomic_write(&ct->state, sizeof(ct->state)); instrumentation_end(); WRITE_ONCE(ct->nesting, 0); /* Avoid irq-access tearing. */ // RCU is watching here ... ct_kernel_exit_state(offset); // ... but is no longer watching here. rcu_task_exit(); } /* * Exit an RCU extended quiescent state, which can be either the * idle loop or adaptive-tickless usermode execution. * * We crowbar the ->nmi_nesting field to CT_NESTING_IRQ_NONIDLE to * allow for the possibility of usermode upcalls messing up our count of * interrupt nesting level during the busy period that is just now starting. */ static void noinstr ct_kernel_enter(bool user, int offset) { struct context_tracking *ct = this_cpu_ptr(&context_tracking); long oldval; WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !raw_irqs_disabled()); oldval = ct_nesting(); WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && oldval < 0); if (oldval) { // RCU was already watching, so just do accounting and leave. ct->nesting++; return; } rcu_task_enter(); // RCU is not watching here ... ct_kernel_enter_state(offset); // ... but is watching here. instrumentation_begin(); // instrumentation for the noinstr ct_kernel_enter_state() instrument_atomic_write(&ct->state, sizeof(ct->state)); trace_rcu_watching(TPS("Start"), ct_nesting(), 1, ct_rcu_watching()); WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current)); WRITE_ONCE(ct->nesting, 1); WARN_ON_ONCE(ct_nmi_nesting()); WRITE_ONCE(ct->nmi_nesting, CT_NESTING_IRQ_NONIDLE); instrumentation_end(); } /** * ct_nmi_exit - inform RCU of exit from NMI context * * If we are returning from the outermost NMI handler that interrupted an * RCU-idle period, update ct->state and ct->nmi_nesting * to let the RCU grace-period handling know that the CPU is back to * being RCU-idle. * * If you add or remove a call to ct_nmi_exit(), be sure to test * with CONFIG_RCU_EQS_DEBUG=y. */ void noinstr ct_nmi_exit(void) { struct context_tracking *ct = this_cpu_ptr(&context_tracking); instrumentation_begin(); /* * Check for ->nmi_nesting underflow and bad CT state. * (We are exiting an NMI handler, so RCU better be paying attention * to us!) */ WARN_ON_ONCE(ct_nmi_nesting() <= 0); WARN_ON_ONCE(!rcu_is_watching_curr_cpu()); /* * If the nesting level is not 1, the CPU wasn't RCU-idle, so * leave it in non-RCU-idle state. */ if (ct_nmi_nesting() != 1) { trace_rcu_watching(TPS("--="), ct_nmi_nesting(), ct_nmi_nesting() - 2, ct_rcu_watching()); WRITE_ONCE(ct->nmi_nesting, /* No store tearing. */ ct_nmi_nesting() - 2); instrumentation_end(); return; } /* This NMI interrupted an RCU-idle CPU, restore RCU-idleness. */ trace_rcu_watching(TPS("Endirq"), ct_nmi_nesting(), 0, ct_rcu_watching()); WRITE_ONCE(ct->nmi_nesting, 0); /* Avoid store tearing. */ // instrumentation for the noinstr ct_kernel_exit_state() instrument_atomic_write(&ct->state, sizeof(ct->state)); instrumentation_end(); // RCU is watching here ... ct_kernel_exit_state(CT_RCU_WATCHING); // ... but is no longer watching here. if (!in_nmi()) rcu_task_exit(); } /** * ct_nmi_enter - inform RCU of entry to NMI context * * If the CPU was idle from RCU's viewpoint, update ct->state and * ct->nmi_nesting to let the RCU grace-period handling know * that the CPU is active. This implementation permits nested NMIs, as * long as the nesting level does not overflow an int. (You will probably * run out of stack space first.) * * If you add or remove a call to ct_nmi_enter(), be sure to test * with CONFIG_RCU_EQS_DEBUG=y. */ void noinstr ct_nmi_enter(void) { long incby = 2; struct context_tracking *ct = this_cpu_ptr(&context_tracking); /* Complain about underflow. */ WARN_ON_ONCE(ct_nmi_nesting() < 0); /* * If idle from RCU viewpoint, atomically increment CT state * to mark non-idle and increment ->nmi_nesting by one. * Otherwise, increment ->nmi_nesting by two. This means * if ->nmi_nesting is equal to one, we are guaranteed * to be in the outermost NMI handler that interrupted an RCU-idle * period (observation due to Andy Lutomirski). */ if (!rcu_is_watching_curr_cpu()) { if (!in_nmi()) rcu_task_enter(); // RCU is not watching here ... ct_kernel_enter_state(CT_RCU_WATCHING); // ... but is watching here. instrumentation_begin(); // instrumentation for the noinstr rcu_is_watching_curr_cpu() instrument_atomic_read(&ct->state, sizeof(ct->state)); // instrumentation for the noinstr ct_kernel_enter_state() instrument_atomic_write(&ct->state, sizeof(ct->state)); incby = 1; } else if (!in_nmi()) { instrumentation_begin(); rcu_irq_enter_check_tick(); } else { instrumentation_begin(); } trace_rcu_watching(incby == 1 ? TPS("Startirq") : TPS("++="), ct_nmi_nesting(), ct_nmi_nesting() + incby, ct_rcu_watching()); instrumentation_end(); WRITE_ONCE(ct->nmi_nesting, /* Prevent store tearing. */ ct_nmi_nesting() + incby); barrier(); } /** * ct_idle_enter - inform RCU that current CPU is entering idle * * Enter idle mode, in other words, -leave- the mode in which RCU * read-side critical sections can occur. (Though RCU read-side * critical sections can occur in irq handlers in idle, a possibility * handled by irq_enter() and irq_exit().) * * If you add or remove a call to ct_idle_enter(), be sure to test with * CONFIG_RCU_EQS_DEBUG=y. */ void noinstr ct_idle_enter(void) { WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !raw_irqs_disabled()); ct_kernel_exit(false, CT_RCU_WATCHING + CT_STATE_IDLE); } EXPORT_SYMBOL_GPL(ct_idle_enter); /** * ct_idle_exit - inform RCU that current CPU is leaving idle * * Exit idle mode, in other words, -enter- the mode in which RCU * read-side critical sections can occur. * * If you add or remove a call to ct_idle_exit(), be sure to test with * CONFIG_RCU_EQS_DEBUG=y. */ void noinstr ct_idle_exit(void) { unsigned long flags; raw_local_irq_save(flags); ct_kernel_enter(false, CT_RCU_WATCHING - CT_STATE_IDLE); raw_local_irq_restore(flags); } EXPORT_SYMBOL_GPL(ct_idle_exit); /** * ct_irq_enter - inform RCU that current CPU is entering irq away from idle * * Enter an interrupt handler, which might possibly result in exiting * idle mode, in other words, entering the mode in which read-side critical * sections can occur. The caller must have disabled interrupts. * * Note that the Linux kernel is fully capable of entering an interrupt * handler that it never exits, for example when doing upcalls to user mode! * This code assumes that the idle loop never does upcalls to user mode. * If your architecture's idle loop does do upcalls to user mode (or does * anything else that results in unbalanced calls to the irq_enter() and * irq_exit() functions), RCU will give you what you deserve, good and hard. * But very infrequently and irreproducibly. * * Use things like work queues to work around this limitation. * * You have been warned. * * If you add or remove a call to ct_irq_enter(), be sure to test with * CONFIG_RCU_EQS_DEBUG=y. */ noinstr void ct_irq_enter(void) { lockdep_assert_irqs_disabled(); ct_nmi_enter(); } /** * ct_irq_exit - inform RCU that current CPU is exiting irq towards idle * * Exit from an interrupt handler, which might possibly result in entering * idle mode, in other words, leaving the mode in which read-side critical * sections can occur. The caller must have disabled interrupts. * * This code assumes that the idle loop never does anything that might * result in unbalanced calls to irq_enter() and irq_exit(). If your * architecture's idle loop violates this assumption, RCU will give you what * you deserve, good and hard. But very infrequently and irreproducibly. * * Use things like work queues to work around this limitation. * * You have been warned. * * If you add or remove a call to ct_irq_exit(), be sure to test with * CONFIG_RCU_EQS_DEBUG=y. */ noinstr void ct_irq_exit(void) { lockdep_assert_irqs_disabled(); ct_nmi_exit(); } /* * Wrapper for ct_irq_enter() where interrupts are enabled. * * If you add or remove a call to ct_irq_enter_irqson(), be sure to test * with CONFIG_RCU_EQS_DEBUG=y. */ void ct_irq_enter_irqson(void) { unsigned long flags; local_irq_save(flags); ct_irq_enter(); local_irq_restore(flags); } /* * Wrapper for ct_irq_exit() where interrupts are enabled. * * If you add or remove a call to ct_irq_exit_irqson(), be sure to test * with CONFIG_RCU_EQS_DEBUG=y. */ void ct_irq_exit_irqson(void) { unsigned long flags; local_irq_save(flags); ct_irq_exit(); local_irq_restore(flags); } #else static __always_inline void ct_kernel_exit(bool user, int offset) { } static __always_inline void ct_kernel_enter(bool user, int offset) { } #endif /* #ifdef CONFIG_CONTEXT_TRACKING_IDLE */ #ifdef CONFIG_CONTEXT_TRACKING_USER #define CREATE_TRACE_POINTS #include <trace/events/context_tracking.h> DEFINE_STATIC_KEY_FALSE_RO(context_tracking_key); EXPORT_SYMBOL_GPL(context_tracking_key); static noinstr bool context_tracking_recursion_enter(void) { int recursion; recursion = __this_cpu_inc_return(context_tracking.recursion); if (recursion == 1) return true; WARN_ONCE((recursion < 1), "Invalid context tracking recursion value %d\n", recursion); __this_cpu_dec(context_tracking.recursion); return false; } static __always_inline void context_tracking_recursion_exit(void) { __this_cpu_dec(context_tracking.recursion); } /** * __ct_user_enter - Inform the context tracking that the CPU is going * to enter user or guest space mode. * * @state: userspace context-tracking state to enter. * * This function must be called right before we switch from the kernel * to user or guest space, when it's guaranteed the remaining kernel * instructions to execute won't use any RCU read side critical section * because this function sets RCU in extended quiescent state. */ void noinstr __ct_user_enter(enum ctx_state state) { struct context_tracking *ct = this_cpu_ptr(&context_tracking); lockdep_assert_irqs_disabled(); /* Kernel threads aren't supposed to go to userspace */ WARN_ON_ONCE(!current->mm); if (!context_tracking_recursion_enter()) return; if (__ct_state() != state) { if (ct->active) { /* * At this stage, only low level arch entry code remains and * then we'll run in userspace. We can assume there won't be * any RCU read-side critical section until the next call to * user_exit() or ct_irq_enter(). Let's remove RCU's dependency * on the tick. */ if (state == CT_STATE_USER) { instrumentation_begin(); trace_user_enter(0); vtime_user_enter(current); instrumentation_end(); } /* * Other than generic entry implementation, we may be past the last * rescheduling opportunity in the entry code. Trigger a self IPI * that will fire and reschedule once we resume in user/guest mode. */ rcu_irq_work_resched(); /* * Enter RCU idle mode right before resuming userspace. No use of RCU * is permitted between this call and rcu_eqs_exit(). This way the * CPU doesn't need to maintain the tick for RCU maintenance purposes * when the CPU runs in userspace. */ ct_kernel_exit(true, CT_RCU_WATCHING + state); /* * Special case if we only track user <-> kernel transitions for tickless * cputime accounting but we don't support RCU extended quiescent state. * In this we case we don't care about any concurrency/ordering. */ if (!IS_ENABLED(CONFIG_CONTEXT_TRACKING_IDLE)) raw_atomic_set(&ct->state, state); } else { /* * Even if context tracking is disabled on this CPU, because it's outside * the full dynticks mask for example, we still have to keep track of the * context transitions and states to prevent inconsistency on those of * other CPUs. * If a task triggers an exception in userspace, sleep on the exception * handler and then migrate to another CPU, that new CPU must know where * the exception returns by the time we call exception_exit(). * This information can only be provided by the previous CPU when it called * exception_enter(). * OTOH we can spare the calls to vtime and RCU when context_tracking.active * is false because we know that CPU is not tickless. */ if (!IS_ENABLED(CONFIG_CONTEXT_TRACKING_IDLE)) { /* Tracking for vtime only, no concurrent RCU EQS accounting */ raw_atomic_set(&ct->state, state); } else { /* * Tracking for vtime and RCU EQS. Make sure we don't race * with NMIs. OTOH we don't care about ordering here since * RCU only requires CT_RCU_WATCHING increments to be fully * ordered. */ raw_atomic_add(state, &ct->state); } } } context_tracking_recursion_exit(); } EXPORT_SYMBOL_GPL(__ct_user_enter); /* * OBSOLETE: * This function should be noinstr but the below local_irq_restore() is * unsafe because it involves illegal RCU uses through tracing and lockdep. * This is unlikely to be fixed as this function is obsolete. The preferred * way is to call __context_tracking_enter() through user_enter_irqoff() * or context_tracking_guest_enter(). It should be the arch entry code * responsibility to call into context tracking with IRQs disabled. */ void ct_user_enter(enum ctx_state state) { unsigned long flags; /* * Some contexts may involve an exception occuring in an irq, * leading to that nesting: * ct_irq_enter() rcu_eqs_exit(true) rcu_eqs_enter(true) ct_irq_exit() * This would mess up the dyntick_nesting count though. And rcu_irq_*() * helpers are enough to protect RCU uses inside the exception. So * just return immediately if we detect we are in an IRQ. */ if (in_interrupt()) return; local_irq_save(flags); __ct_user_enter(state); local_irq_restore(flags); } NOKPROBE_SYMBOL(ct_user_enter); EXPORT_SYMBOL_GPL(ct_user_enter); /** * user_enter_callable() - Unfortunate ASM callable version of user_enter() for * archs that didn't manage to check the context tracking * static key from low level code. * * This OBSOLETE function should be noinstr but it unsafely calls * local_irq_restore(), involving illegal RCU uses through tracing and lockdep. * This is unlikely to be fixed as this function is obsolete. The preferred * way is to call user_enter_irqoff(). It should be the arch entry code * responsibility to call into context tracking with IRQs disabled. */ void user_enter_callable(void) { user_enter(); } NOKPROBE_SYMBOL(user_enter_callable); /** * __ct_user_exit - Inform the context tracking that the CPU is * exiting user or guest mode and entering the kernel. * * @state: userspace context-tracking state being exited from. * * This function must be called after we entered the kernel from user or * guest space before any use of RCU read side critical section. This * potentially include any high level kernel code like syscalls, exceptions, * signal handling, etc... * * This call supports re-entrancy. This way it can be called from any exception * handler without needing to know if we came from userspace or not. */ void noinstr __ct_user_exit(enum ctx_state state) { struct context_tracking *ct = this_cpu_ptr(&context_tracking); if (!context_tracking_recursion_enter()) return; if (__ct_state() == state) { if (ct->active) { /* * Exit RCU idle mode while entering the kernel because it can * run a RCU read side critical section anytime. */ ct_kernel_enter(true, CT_RCU_WATCHING - state); if (state == CT_STATE_USER) { instrumentation_begin(); vtime_user_exit(current); trace_user_exit(0); instrumentation_end(); } /* * Special case if we only track user <-> kernel transitions for tickless * cputime accounting but we don't support RCU extended quiescent state. * In this we case we don't care about any concurrency/ordering. */ if (!IS_ENABLED(CONFIG_CONTEXT_TRACKING_IDLE)) raw_atomic_set(&ct->state, CT_STATE_KERNEL); } else { if (!IS_ENABLED(CONFIG_CONTEXT_TRACKING_IDLE)) { /* Tracking for vtime only, no concurrent RCU EQS accounting */ raw_atomic_set(&ct->state, CT_STATE_KERNEL); } else { /* * Tracking for vtime and RCU EQS. Make sure we don't race * with NMIs. OTOH we don't care about ordering here since * RCU only requires CT_RCU_WATCHING increments to be fully * ordered. */ raw_atomic_sub(state, &ct->state); } } } context_tracking_recursion_exit(); } EXPORT_SYMBOL_GPL(__ct_user_exit); /* * OBSOLETE: * This function should be noinstr but the below local_irq_save() is * unsafe because it involves illegal RCU uses through tracing and lockdep. * This is unlikely to be fixed as this function is obsolete. The preferred * way is to call __context_tracking_exit() through user_exit_irqoff() * or context_tracking_guest_exit(). It should be the arch entry code * responsibility to call into context tracking with IRQs disabled. */ void ct_user_exit(enum ctx_state state) { unsigned long flags; if (in_interrupt()) return; local_irq_save(flags); __ct_user_exit(state); local_irq_restore(flags); } NOKPROBE_SYMBOL(ct_user_exit); EXPORT_SYMBOL_GPL(ct_user_exit); /** * user_exit_callable() - Unfortunate ASM callable version of user_exit() for * archs that didn't manage to check the context tracking * static key from low level code. * * This OBSOLETE function should be noinstr but it unsafely calls local_irq_save(), * involving illegal RCU uses through tracing and lockdep. This is unlikely * to be fixed as this function is obsolete. The preferred way is to call * user_exit_irqoff(). It should be the arch entry code responsibility to * call into context tracking with IRQs disabled. */ void user_exit_callable(void) { user_exit(); } NOKPROBE_SYMBOL(user_exit_callable); void __init ct_cpu_track_user(int cpu) { static __initdata bool initialized = false; if (!per_cpu(context_tracking.active, cpu)) { per_cpu(context_tracking.active, cpu) = true; static_branch_inc(&context_tracking_key); } if (initialized) return; #ifdef CONFIG_HAVE_TIF_NOHZ /* * Set TIF_NOHZ to init/0 and let it propagate to all tasks through fork * This assumes that init is the only task at this early boot stage. */ set_tsk_thread_flag(&init_task, TIF_NOHZ); #endif WARN_ON_ONCE(!tasklist_empty()); initialized = true; } #ifdef CONFIG_CONTEXT_TRACKING_USER_FORCE void __init context_tracking_init(void) { int cpu; for_each_possible_cpu(cpu) ct_cpu_track_user(cpu); } #endif #endif /* #ifdef CONFIG_CONTEXT_TRACKING_USER */
