/* * kernel/locking/mutex.c * * Mutexes: blocking mutual exclusion locks * * Started by Ingo Molnar: * * Copyright (C) 2004, 2005, 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com> * * Many thanks to Arjan van de Ven, Thomas Gleixner, Steven Rostedt and * David Howells for suggestions and improvements. * * - Adaptive spinning for mutexes by Peter Zijlstra. (Ported to mainline * from the -rt tree, where it was originally implemented for rtmutexes * by Steven Rostedt, based on work by Gregory Haskins, Peter Morreale * and Sven Dietrich. * * Also see Documentation/mutex-design.txt. */#include <linux/mutex.h>#include <linux/ww_mutex.h>#include <linux/sched.h>#include <linux/sched/rt.h>#include <linux/export.h>#include <linux/spinlock.h>#include <linux/interrupt.h>#include <linux/debug_locks.h>#include "mcs_spinlock.h" /* * In the DEBUG case we are using the "NULL fastpath" for mutexes, * which forces all calls into the slowpath: */#ifdef CONFIG_DEBUG_MUTEXES# include "mutex-debug.h"# include <asm-generic/mutex-null.h>/* * Must be 0 for the debug case so we do not do the unlock outside of the * wait_lock region. debug_mutex_unlock() will do the actual unlock in this * case. */# undef __mutex_slowpath_needs_to_unlock# define __mutex_slowpath_needs_to_unlock() 0#else# include "mutex.h"# include <asm/mutex.h>#endif void__mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key){ atomic_set(&lock->count, 1); spin_lock_init(&lock->wait_lock); INIT_LIST_HEAD(&lock->wait_list); mutex_clear_owner(lock);#ifdef CONFIG_MUTEX_SPIN_ON_OWNER osq_lock_init(&lock->osq);#endif debug_mutex_init(lock, name, key);} EXPORT_SYMBOL(__mutex_init); #ifndef CONFIG_DEBUG_LOCK_ALLOC/* * We split the mutex lock/unlock logic into separate fastpath and * slowpath functions, to reduce the register pressure on the fastpath. * We also put the fastpath first in the kernel image, to make sure the * branch is predicted by the CPU as default-untaken. */__visible void __sched __mutex_lock_slowpath(atomic_t *lock_count); /** * mutex_lock - acquire the mutex * @lock: the mutex to be acquired * * Lock the mutex exclusively for this task. If the mutex is not * available right now, it will sleep until it can get it. * * The mutex must later on be released by the same task that * acquired it. Recursive locking is not allowed. The task * may not exit without first unlocking the mutex. Also, kernel * memory where the mutex resides mutex must not be freed with * the mutex still locked. The mutex must first be initialized * (or statically defined) before it can be locked. memset()-ing * the mutex to 0 is not allowed. * * ( The CONFIG_DEBUG_MUTEXES .config option turns on debugging * checks that will enforce the restrictions and will also do * deadlock debugging. ) * * This function is similar to (but not equivalent to) down(). */void __sched mutex_lock(struct mutex *lock){ might_sleep(); /* * The locking fastpath is the 1->0 transition from * 'unlocked' into 'locked' state. */ __mutex_fastpath_lock(&lock->count, __mutex_lock_slowpath); mutex_set_owner(lock);} EXPORT_SYMBOL(mutex_lock);#endif #ifdef CONFIG_MUTEX_SPIN_ON_OWNER/* * In order to avoid a stampede of mutex spinners from acquiring the mutex * more or less simultaneously, the spinners need to acquire a MCS lock * first before spinning on the owner field. * */ /* * Mutex spinning code migrated from kernel/sched/core.c */ static inline bool owner_running(struct mutex *lock, struct task_struct *owner){ if (lock->owner != owner) return false; /* * Ensure we emit the owner->on_cpu, dereference _after_ checking * lock->owner still matches owner, if that fails, owner might * point to free()d memory, if it still matches, the rcu_read_lock() * ensures the memory stays valid. */ barrier(); return owner->on_cpu;} /* * Look out! "owner" is an entirely speculative pointer * access and not reliable. */static noinlineint mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner){ rcu_read_lock(); while (owner_running(lock, owner)) { if (need_resched()) break; cpu_relax_lowlatency(); } rcu_read_unlock(); /* * We break out the loop above on need_resched() and when the * owner changed, which is a sign for heavy contention. Return * success only when lock->owner is NULL. */ return lock->owner == NULL;} /* * Initial check for entering the mutex spinning loop */static inline int mutex_can_spin_on_owner(struct mutex *lock){ struct task_struct *owner; int retval = 1; if (need_resched()) return 0; rcu_read_lock(); owner = ACCESS_ONCE(lock->owner); if (owner) retval = owner->on_cpu; rcu_read_unlock(); /* * if lock->owner is not set, the mutex owner may have just acquired * it and not set the owner yet or the mutex has been released. */ return retval;}#endif __visible __used noinlinevoid __sched __mutex_unlock_slowpath(atomic_t *lock_count); /** * mutex_unlock - release the mutex * @lock: the mutex to be released * * Unlock a mutex that has been locked by this task previously. * * This function must not be used in interrupt context. Unlocking * of a not locked mutex is not allowed. * * This function is similar to (but not equivalent to) up(). */void __sched mutex_unlock(struct mutex *lock){ /* * The unlocking fastpath is the 0->1 transition from 'locked' * into 'unlocked' state: */#ifndef CONFIG_DEBUG_MUTEXES /* * When debugging is enabled we must not clear the owner before time, * the slow path will always be taken, and that clears the owner field * after verifying that it was indeed current. */ mutex_clear_owner(lock);#endif __mutex_fastpath_unlock(&lock->count, __mutex_unlock_slowpath);} EXPORT_SYMBOL(mutex_unlock); /** * ww_mutex_unlock - release the w/w mutex * @lock: the mutex to be released * * Unlock a mutex that has been locked by this task previously with any of the * ww_mutex_lock* functions (with or without an acquire context). It is * forbidden to release the locks after releasing the acquire context. * * This function must not be used in interrupt context. Unlocking * of a unlocked mutex is not allowed. */void __sched ww_mutex_unlock(struct ww_mutex *lock){ /* * The unlocking fastpath is the 0->1 transition from 'locked' * into 'unlocked' state: */ if (lock->ctx) {#ifdef CONFIG_DEBUG_MUTEXES DEBUG_LOCKS_WARN_ON(!lock->ctx->acquired);#endif if (lock->ctx->acquired > 0) lock->ctx->acquired--; lock->ctx = NULL; } #ifndef CONFIG_DEBUG_MUTEXES /* * When debugging is enabled we must not clear the owner before time, * the slow path will always be taken, and that clears the owner field * after verifying that it was indeed current. */ mutex_clear_owner(&lock->base);#endif __mutex_fastpath_unlock(&lock->base.count, __mutex_unlock_slowpath);}EXPORT_SYMBOL(ww_mutex_unlock); static inline int __sched__mutex_lock_check_stamp(struct mutex *lock, struct ww_acquire_ctx *ctx){ struct ww_mutex *ww = container_of(lock, struct ww_mutex, base); struct ww_acquire_ctx *hold_ctx = ACCESS_ONCE(ww->ctx); if (!hold_ctx) return 0; if (unlikely(ctx == hold_ctx)) return -EALREADY; if (ctx->stamp - hold_ctx->stamp <= LONG_MAX && (ctx->stamp != hold_ctx->stamp || ctx > hold_ctx)) {#ifdef CONFIG_DEBUG_MUTEXES DEBUG_LOCKS_WARN_ON(ctx->contending_lock); ctx->contending_lock = ww;#endif return -EDEADLK; } return 0;} static __always_inline void ww_mutex_lock_acquired(struct ww_mutex *ww, struct ww_acquire_ctx *ww_ctx){#ifdef CONFIG_DEBUG_MUTEXES /* * If this WARN_ON triggers, you used ww_mutex_lock to acquire, * but released with a normal mutex_unlock in this call. * * This should never happen, always use ww_mutex_unlock. */ DEBUG_LOCKS_WARN_ON(ww->ctx); /* * Not quite done after calling ww_acquire_done() ? */ DEBUG_LOCKS_WARN_ON(ww_ctx->done_acquire); if (ww_ctx->contending_lock) { /* * After -EDEADLK you tried to * acquire a different ww_mutex? Bad! */ DEBUG_LOCKS_WARN_ON(ww_ctx->contending_lock != ww); /* * You called ww_mutex_lock after receiving -EDEADLK, * but 'forgot' to unlock everything else first? */ DEBUG_LOCKS_WARN_ON(ww_ctx->acquired > 0); ww_ctx->contending_lock = NULL; } /* * Naughty, using a different class will lead to undefined behavior! */ DEBUG_LOCKS_WARN_ON(ww_ctx->ww_class != ww->ww_class);#endif ww_ctx->acquired++;} /* * after acquiring lock with fastpath or when we lost out in contested * slowpath, set ctx and wake up any waiters so they can recheck. * * This function is never called when CONFIG_DEBUG_LOCK_ALLOC is set, * as the fastpath and opportunistic spinning are disabled in that case. */static __always_inline voidww_mutex_set_context_fastpath(struct ww_mutex *lock, struct ww_acquire_ctx *ctx){ unsigned long flags; struct mutex_waiter *cur; ww_mutex_lock_acquired(lock, ctx); lock->ctx = ctx; /* * The lock->ctx update should be visible on all cores before * the atomic read is done, otherwise contended waiters might be * missed. The contended waiters will either see ww_ctx == NULL * and keep spinning, or it will acquire wait_lock, add itself * to waiter list and sleep. */ smp_mb(); /* ^^^ */ /* * Check if lock is contended, if not there is nobody to wake up */ if (likely(atomic_read(&lock->base.count) == 0)) return; /* * Uh oh, we raced in fastpath, wake up everyone in this case, * so they can see the new lock->ctx. */ spin_lock_mutex(&lock->base.wait_lock, flags); list_for_each_entry(cur, &lock->base.wait_list, list) { debug_mutex_wake_waiter(&lock->base, cur); wake_up_process(cur->task); } spin_unlock_mutex(&lock->base.wait_lock, flags);} /* * Lock a mutex (possibly interruptible), slowpath: */static __always_inline int __sched__mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, struct lockdep_map *nest_lock, unsigned long ip, struct ww_acquire_ctx *ww_ctx, const bool use_ww_ctx){ struct task_struct *task = current; struct mutex_waiter waiter; unsigned long flags; int ret; preempt_disable(); mutex_acquire_nest(&lock->dep_map, subclass, 0, nest_lock, ip); #ifdef CONFIG_MUTEX_SPIN_ON_OWNER /* * Optimistic spinning. * * We try to spin for acquisition when we find that the lock owner * is currently running on a (different) CPU and while we don't * need to reschedule. The rationale is that if the lock owner is * running, it is likely to release the lock soon. * * Since this needs the lock owner, and this mutex implementation * doesn't track the owner atomically in the lock field, we need to * track it non-atomically. * * We can't do this for DEBUG_MUTEXES because that relies on wait_lock * to serialize everything. * * The mutex spinners are queued up using MCS lock so that only one * spinner can compete for the mutex. However, if mutex spinning isn't * going to happen, there is no point in going through the lock/unlock * overhead. */ if (!mutex_can_spin_on_owner(lock)) goto slowpath; if (!osq_lock(&lock->osq)) goto slowpath; for (;;) { struct task_struct *owner; if (use_ww_ctx && ww_ctx->acquired > 0) { struct ww_mutex *ww; ww = container_of(lock, struct ww_mutex, base); /* * If ww->ctx is set the contents are undefined, only * by acquiring wait_lock there is a guarantee that * they are not invalid when reading. * * As such, when deadlock detection needs to be * performed the optimistic spinning cannot be done. */ if (ACCESS_ONCE(ww->ctx)) break; } /* * If there's an owner, wait for it to either * release the lock or go to sleep. */ owner = ACCESS_ONCE(lock->owner); if (owner && !mutex_spin_on_owner(lock, owner)) break; /* Try to acquire the mutex if it is unlocked. */ if (!mutex_is_locked(lock) && (atomic_cmpxchg(&lock->count, 1, 0) == 1)) { lock_acquired(&lock->dep_map, ip); if (use_ww_ctx) { struct ww_mutex *ww; ww = container_of(lock, struct ww_mutex, base); ww_mutex_set_context_fastpath(ww, ww_ctx); } mutex_set_owner(lock); osq_unlock(&lock->osq); preempt_enable(); return 0; } /* * When there's no owner, we might have preempted between the * owner acquiring the lock and setting the owner field. If * we're an RT task that will live-lock because we won't let * the owner complete. */ if (!owner && (need_resched() || rt_task(task))) break; /* * The cpu_relax() call is a compiler barrier which forces * everything in this loop to be re-loaded. We don't need * memory barriers as we'll eventually observe the right * values at the cost of a few extra spins. */ cpu_relax_lowlatency(); } osq_unlock(&lock->osq);slowpath: /* * If we fell out of the spin path because of need_resched(), * reschedule now, before we try-lock the mutex. This avoids getting * scheduled out right after we obtained the mutex. */ if (need_resched()) schedule_preempt_disabled();#endif spin_lock_mutex(&lock->wait_lock, flags); /* * Once more, try to acquire the lock. Only try-lock the mutex if * it is unlocked to reduce unnecessary xchg() operations. */ if (!mutex_is_locked(lock) && (atomic_xchg(&lock->count, 0) == 1)) goto skip_wait; debug_mutex_lock_common(lock, &waiter); debug_mutex_add_waiter(lock, &waiter, task_thread_info(task)); /* add waiting tasks to the end of the waitqueue (FIFO): */ list_add_tail(&waiter.list, &lock->wait_list); waiter.task = task; lock_contended(&lock->dep_map, ip); for (;;) { /* * Lets try to take the lock again - this is needed even if * we get here for the first time (shortly after failing toLines 1–500 of 931. Load more
/* * kernel/locking/mutex.c * * Mutexes: blocking mutual exclusion locks * * Started by Ingo Molnar: * * Copyright (C) 2004, 2005, 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com> * * Many thanks to Arjan van de Ven, Thomas Gleixner, Steven Rostedt and * David Howells for suggestions and improvements. * * - Adaptive spinning for mutexes by Peter Zijlstra. (Ported to mainline * from the -rt tree, where it was originally implemented for rtmutexes * by Steven Rostedt, based on work by Gregory Haskins, Peter Morreale * and Sven Dietrich. * * Also see Documentation/mutex-design.txt. */ #include <linux/mutex.h> #include <linux/ww_mutex.h> #include <linux/sched.h> #include <linux/sched/rt.h> #include <linux/export.h> #include <linux/spinlock.h> #include <linux/interrupt.h> #include <linux/debug_locks.h> #include "mcs_spinlock.h" /* * In the DEBUG case we are using the "NULL fastpath" for mutexes, * which forces all calls into the slowpath: */ #ifdef CONFIG_DEBUG_MUTEXES # include "mutex-debug.h" # include <asm-generic/mutex-null.h> /* * Must be 0 for the debug case so we do not do the unlock outside of the * wait_lock region. debug_mutex_unlock() will do the actual unlock in this * case. */ # undef __mutex_slowpath_needs_to_unlock # define __mutex_slowpath_needs_to_unlock() 0 #else # include "mutex.h" # include <asm/mutex.h> #endif void __mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key) { atomic_set(&lock->count, 1); spin_lock_init(&lock->wait_lock); INIT_LIST_HEAD(&lock->wait_list); mutex_clear_owner(lock); #ifdef CONFIG_MUTEX_SPIN_ON_OWNER osq_lock_init(&lock->osq); #endif debug_mutex_init(lock, name, key); } EXPORT_SYMBOL(__mutex_init); #ifndef CONFIG_DEBUG_LOCK_ALLOC /* * We split the mutex lock/unlock logic into separate fastpath and * slowpath functions, to reduce the register pressure on the fastpath. * We also put the fastpath first in the kernel image, to make sure the * branch is predicted by the CPU as default-untaken. */ __visible void __sched __mutex_lock_slowpath(atomic_t *lock_count); /** * mutex_lock - acquire the mutex * @lock: the mutex to be acquired * * Lock the mutex exclusively for this task. If the mutex is not * available right now, it will sleep until it can get it. * * The mutex must later on be released by the same task that * acquired it. Recursive locking is not allowed. The task * may not exit without first unlocking the mutex. Also, kernel * memory where the mutex resides mutex must not be freed with * the mutex still locked. The mutex must first be initialized * (or statically defined) before it can be locked. memset()-ing * the mutex to 0 is not allowed. * * ( The CONFIG_DEBUG_MUTEXES .config option turns on debugging * checks that will enforce the restrictions and will also do * deadlock debugging. ) * * This function is similar to (but not equivalent to) down(). */ void __sched mutex_lock(struct mutex *lock) { might_sleep(); /* * The locking fastpath is the 1->0 transition from * 'unlocked' into 'locked' state. */ __mutex_fastpath_lock(&lock->count, __mutex_lock_slowpath); mutex_set_owner(lock); } EXPORT_SYMBOL(mutex_lock); #endif #ifdef CONFIG_MUTEX_SPIN_ON_OWNER /* * In order to avoid a stampede of mutex spinners from acquiring the mutex * more or less simultaneously, the spinners need to acquire a MCS lock * first before spinning on the owner field. * */ /* * Mutex spinning code migrated from kernel/sched/core.c */ static inline bool owner_running(struct mutex *lock, struct task_struct *owner) { if (lock->owner != owner) return false; /* * Ensure we emit the owner->on_cpu, dereference _after_ checking * lock->owner still matches owner, if that fails, owner might * point to free()d memory, if it still matches, the rcu_read_lock() * ensures the memory stays valid. */ barrier(); return owner->on_cpu; } /* * Look out! "owner" is an entirely speculative pointer * access and not reliable. */ static noinline int mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner) { rcu_read_lock(); while (owner_running(lock, owner)) { if (need_resched()) break; cpu_relax_lowlatency(); } rcu_read_unlock(); /* * We break out the loop above on need_resched() and when the * owner changed, which is a sign for heavy contention. Return * success only when lock->owner is NULL. */ return lock->owner == NULL; } /* * Initial check for entering the mutex spinning loop */ static inline int mutex_can_spin_on_owner(struct mutex *lock) { struct task_struct *owner; int retval = 1; if (need_resched()) return 0; rcu_read_lock(); owner = ACCESS_ONCE(lock->owner); if (owner) retval = owner->on_cpu; rcu_read_unlock(); /* * if lock->owner is not set, the mutex owner may have just acquired * it and not set the owner yet or the mutex has been released. */ return retval; } #endif __visible __used noinline void __sched __mutex_unlock_slowpath(atomic_t *lock_count); /** * mutex_unlock - release the mutex * @lock: the mutex to be released * * Unlock a mutex that has been locked by this task previously. * * This function must not be used in interrupt context. Unlocking * of a not locked mutex is not allowed. * * This function is similar to (but not equivalent to) up(). */ void __sched mutex_unlock(struct mutex *lock) { /* * The unlocking fastpath is the 0->1 transition from 'locked' * into 'unlocked' state: */ #ifndef CONFIG_DEBUG_MUTEXES /* * When debugging is enabled we must not clear the owner before time, * the slow path will always be taken, and that clears the owner field * after verifying that it was indeed current. */ mutex_clear_owner(lock); #endif __mutex_fastpath_unlock(&lock->count, __mutex_unlock_slowpath); } EXPORT_SYMBOL(mutex_unlock); /** * ww_mutex_unlock - release the w/w mutex * @lock: the mutex to be released * * Unlock a mutex that has been locked by this task previously with any of the * ww_mutex_lock* functions (with or without an acquire context). It is * forbidden to release the locks after releasing the acquire context. * * This function must not be used in interrupt context. Unlocking * of a unlocked mutex is not allowed. */ void __sched ww_mutex_unlock(struct ww_mutex *lock) { /* * The unlocking fastpath is the 0->1 transition from 'locked' * into 'unlocked' state: */ if (lock->ctx) { #ifdef CONFIG_DEBUG_MUTEXES DEBUG_LOCKS_WARN_ON(!lock->ctx->acquired); #endif if (lock->ctx->acquired > 0) lock->ctx->acquired--; lock->ctx = NULL; } #ifndef CONFIG_DEBUG_MUTEXES /* * When debugging is enabled we must not clear the owner before time, * the slow path will always be taken, and that clears the owner field * after verifying that it was indeed current. */ mutex_clear_owner(&lock->base); #endif __mutex_fastpath_unlock(&lock->base.count, __mutex_unlock_slowpath); } EXPORT_SYMBOL(ww_mutex_unlock); static inline int __sched __mutex_lock_check_stamp(struct mutex *lock, struct ww_acquire_ctx *ctx) { struct ww_mutex *ww = container_of(lock, struct ww_mutex, base); struct ww_acquire_ctx *hold_ctx = ACCESS_ONCE(ww->ctx); if (!hold_ctx) return 0; if (unlikely(ctx == hold_ctx)) return -EALREADY; if (ctx->stamp - hold_ctx->stamp <= LONG_MAX && (ctx->stamp != hold_ctx->stamp || ctx > hold_ctx)) { #ifdef CONFIG_DEBUG_MUTEXES DEBUG_LOCKS_WARN_ON(ctx->contending_lock); ctx->contending_lock = ww; #endif return -EDEADLK; } return 0; } static __always_inline void ww_mutex_lock_acquired(struct ww_mutex *ww, struct ww_acquire_ctx *ww_ctx) { #ifdef CONFIG_DEBUG_MUTEXES /* * If this WARN_ON triggers, you used ww_mutex_lock to acquire, * but released with a normal mutex_unlock in this call. * * This should never happen, always use ww_mutex_unlock. */ DEBUG_LOCKS_WARN_ON(ww->ctx); /* * Not quite done after calling ww_acquire_done() ? */ DEBUG_LOCKS_WARN_ON(ww_ctx->done_acquire); if (ww_ctx->contending_lock) { /* * After -EDEADLK you tried to * acquire a different ww_mutex? Bad! */ DEBUG_LOCKS_WARN_ON(ww_ctx->contending_lock != ww); /* * You called ww_mutex_lock after receiving -EDEADLK, * but 'forgot' to unlock everything else first? */ DEBUG_LOCKS_WARN_ON(ww_ctx->acquired > 0); ww_ctx->contending_lock = NULL; } /* * Naughty, using a different class will lead to undefined behavior! */ DEBUG_LOCKS_WARN_ON(ww_ctx->ww_class != ww->ww_class); #endif ww_ctx->acquired++; } /* * after acquiring lock with fastpath or when we lost out in contested * slowpath, set ctx and wake up any waiters so they can recheck. * * This function is never called when CONFIG_DEBUG_LOCK_ALLOC is set, * as the fastpath and opportunistic spinning are disabled in that case. */ static __always_inline void ww_mutex_set_context_fastpath(struct ww_mutex *lock, struct ww_acquire_ctx *ctx) { unsigned long flags; struct mutex_waiter *cur; ww_mutex_lock_acquired(lock, ctx); lock->ctx = ctx; /* * The lock->ctx update should be visible on all cores before * the atomic read is done, otherwise contended waiters might be * missed. The contended waiters will either see ww_ctx == NULL * and keep spinning, or it will acquire wait_lock, add itself * to waiter list and sleep. */ smp_mb(); /* ^^^ */ /* * Check if lock is contended, if not there is nobody to wake up */ if (likely(atomic_read(&lock->base.count) == 0)) return; /* * Uh oh, we raced in fastpath, wake up everyone in this case, * so they can see the new lock->ctx. */ spin_lock_mutex(&lock->base.wait_lock, flags); list_for_each_entry(cur, &lock->base.wait_list, list) { debug_mutex_wake_waiter(&lock->base, cur); wake_up_process(cur->task); } spin_unlock_mutex(&lock->base.wait_lock, flags); } /* * Lock a mutex (possibly interruptible), slowpath: */ static __always_inline int __sched __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, struct lockdep_map *nest_lock, unsigned long ip, struct ww_acquire_ctx *ww_ctx, const bool use_ww_ctx) { struct task_struct *task = current; struct mutex_waiter waiter; unsigned long flags; int ret; preempt_disable(); mutex_acquire_nest(&lock->dep_map, subclass, 0, nest_lock, ip); #ifdef CONFIG_MUTEX_SPIN_ON_OWNER /* * Optimistic spinning. * * We try to spin for acquisition when we find that the lock owner * is currently running on a (different) CPU and while we don't * need to reschedule. The rationale is that if the lock owner is * running, it is likely to release the lock soon. * * Since this needs the lock owner, and this mutex implementation * doesn't track the owner atomically in the lock field, we need to * track it non-atomically. * * We can't do this for DEBUG_MUTEXES because that relies on wait_lock * to serialize everything. * * The mutex spinners are queued up using MCS lock so that only one * spinner can compete for the mutex. However, if mutex spinning isn't * going to happen, there is no point in going through the lock/unlock * overhead. */ if (!mutex_can_spin_on_owner(lock)) goto slowpath; if (!osq_lock(&lock->osq)) goto slowpath; for (;;) { struct task_struct *owner; if (use_ww_ctx && ww_ctx->acquired > 0) { struct ww_mutex *ww; ww = container_of(lock, struct ww_mutex, base); /* * If ww->ctx is set the contents are undefined, only * by acquiring wait_lock there is a guarantee that * they are not invalid when reading. * * As such, when deadlock detection needs to be * performed the optimistic spinning cannot be done. */ if (ACCESS_ONCE(ww->ctx)) break; } /* * If there's an owner, wait for it to either * release the lock or go to sleep. */ owner = ACCESS_ONCE(lock->owner); if (owner && !mutex_spin_on_owner(lock, owner)) break; /* Try to acquire the mutex if it is unlocked. */ if (!mutex_is_locked(lock) && (atomic_cmpxchg(&lock->count, 1, 0) == 1)) { lock_acquired(&lock->dep_map, ip); if (use_ww_ctx) { struct ww_mutex *ww; ww = container_of(lock, struct ww_mutex, base); ww_mutex_set_context_fastpath(ww, ww_ctx); } mutex_set_owner(lock); osq_unlock(&lock->osq); preempt_enable(); return 0; } /* * When there's no owner, we might have preempted between the * owner acquiring the lock and setting the owner field. If * we're an RT task that will live-lock because we won't let * the owner complete. */ if (!owner && (need_resched() || rt_task(task))) break; /* * The cpu_relax() call is a compiler barrier which forces * everything in this loop to be re-loaded. We don't need * memory barriers as we'll eventually observe the right * values at the cost of a few extra spins. */ cpu_relax_lowlatency(); } osq_unlock(&lock->osq); slowpath: /* * If we fell out of the spin path because of need_resched(), * reschedule now, before we try-lock the mutex. This avoids getting * scheduled out right after we obtained the mutex. */ if (need_resched()) schedule_preempt_disabled(); #endif spin_lock_mutex(&lock->wait_lock, flags); /* * Once more, try to acquire the lock. Only try-lock the mutex if * it is unlocked to reduce unnecessary xchg() operations. */ if (!mutex_is_locked(lock) && (atomic_xchg(&lock->count, 0) == 1)) goto skip_wait; debug_mutex_lock_common(lock, &waiter); debug_mutex_add_waiter(lock, &waiter, task_thread_info(task)); /* add waiting tasks to the end of the waitqueue (FIFO): */ list_add_tail(&waiter.list, &lock->wait_list); waiter.task = task; lock_contended(&lock->dep_map, ip); for (;;) { /* * Lets try to take the lock again - this is needed even if * we get here for the first time (shortly after failing to