/* * kernel/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/sched.h> #include <linux/module.h> #include <linux/spinlock.h> #include <linux/interrupt.h> #include <linux/debug_locks.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> #else # include "mutex.h" # include <asm/mutex.h> #endif /*** * mutex_init - initialize the mutex * @lock: the mutex to be initialized * @key: the lock_class_key for the class; used by mutex lock debugging * * Initialize the mutex to unlocked state. * * It is not allowed to initialize an already locked mutex. */ 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); 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. */ static __used noinline 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 static __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); /* * Lock a mutex (possibly interruptible), slowpath: */ static inline int __sched __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, unsigned long ip) { struct task_struct *task = current; struct mutex_waiter waiter; unsigned long flags; preempt_disable(); mutex_acquire(&lock->dep_map, subclass, 0, ip); #if defined(CONFIG_SMP) && !defined(CONFIG_DEBUG_MUTEXES) && \ !defined(CONFIG_HAVE_DEFAULT_NO_SPIN_MUTEXES) /* * Optimistic spinning. * * We try to spin for acquisition when we find that there are no * pending waiters and the lock owner is currently running on a * (different) CPU. * * 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. */ for (;;) { struct thread_info *owner; /* * 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; if (atomic_cmpxchg(&lock->count, 1, 0) == 1) { lock_acquired(&lock->dep_map, ip); mutex_set_owner(lock); 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(); } #endif spin_lock_mutex(&lock->wait_lock, flags); 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; if (atomic_xchg(&lock->count, -1) == 1) goto done; 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 * acquire the lock), to make sure that we get a wakeup once * it's unlocked. Later on, if we sleep, this is the * operation that gives us the lock. We xchg it to -1, so * that when we release the lock, we properly wake up the * other waiters: */ if (atomic_xchg(&lock->count, -1) == 1) break; /* * got a signal? (This code gets eliminated in the * TASK_UNINTERRUPTIBLE case.) */ if (unlikely(signal_pending_state(state, task))) { mutex_remove_waiter(lock, &waiter, task_thread_info(task)); mutex_release(&lock->dep_map, 1, ip); spin_unlock_mutex(&lock->wait_lock, flags); debug_mutex_free_waiter(&waiter); preempt_enable(); return -EINTR; } __set_task_state(task, state); /* didnt get the lock, go to sleep: */ spin_unlock_mutex(&lock->wait_lock, flags); preempt_enable_no_resched(); schedule(); preempt_disable(); spin_lock_mutex(&lock->wait_lock, flags); } done: lock_acquired(&lock->dep_map, ip); /* got the lock - rejoice! */ mutex_remove_waiter(lock, &waiter, current_thread_info()); mutex_set_owner(lock); /* set it to 0 if there are no waiters left: */ if (likely(list_empty(&lock->wait_list))) atomic_set(&lock->count, 0); spin_unlock_mutex(&lock->wait_lock, flags); debug_mutex_free_waiter(&waiter); preempt_enable(); return 0; } #ifdef CONFIG_DEBUG_LOCK_ALLOC void __sched mutex_lock_nested(struct mutex *lock, unsigned int subclass) { might_sleep(); __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, subclass, _RET_IP_); } EXPORT_SYMBOL_GPL(mutex_lock_nested); int __sched mutex_lock_killable_nested(struct mutex *lock, unsigned int subclass) { might_sleep(); return __mutex_lock_common(lock, TASK_KILLABLE, subclass, _RET_IP_); } EXPORT_SYMBOL_GPL(mutex_lock_killable_nested); int __sched mutex_lock_interruptible_nested(struct mutex *lock, unsigned int subclass) { might_sleep(); return __mutex_lock_common(lock, TASK_INTERRUPTIBLE, subclass, _RET_IP_); } EXPORT_SYMBOL_GPL(mutex_lock_interruptible_nested); #endif /* * Release the lock, slowpath: */ static inline void __mutex_unlock_common_slowpath(atomic_t *lock_count, int nested) { struct mutex *lock = container_of(lock_count, struct mutex, count); unsigned long flags; spin_lock_mutex(&lock->wait_lock, flags); mutex_release(&lock->dep_map, nested, _RET_IP_); debug_mutex_unlock(lock); /* * some architectures leave the lock unlocked in the fastpath failure * case, others need to leave it locked. In the later case we have to * unlock it here */ if (__mutex_slowpath_needs_to_unlock()) atomic_set(&lock->count, 1); if (!list_empty(&lock->wait_list)) { /* get the first entry from the wait-list: */ struct mutex_waiter *waiter = list_entry(lock->wait_list.next, struct mutex_waiter, list); debug_mutex_wake_waiter(lock, waiter); wake_up_process(waiter->task); } spin_unlock_mutex(&lock->wait_lock, flags); } /* * Release the lock, slowpath: */ static __used noinline void __mutex_unlock_slowpath(atomic_t *lock_count) { __mutex_unlock_common_slowpath(lock_count, 1); } #ifndef CONFIG_DEBUG_LOCK_ALLOC /* * Here come the less common (and hence less performance-critical) APIs: * mutex_lock_interruptible() and mutex_trylock(). */ static noinline int __sched __mutex_lock_killable_slowpath(atomic_t *lock_count); static noinline int __sched __mutex_lock_interruptible_slowpath(atomic_t *lock_count); /*** * mutex_lock_interruptible - acquire the mutex, interruptable * @lock: the mutex to be acquired * * Lock the mutex like mutex_lock(), and return 0 if the mutex has * been acquired or sleep until the mutex becomes available. If a * signal arrives while waiting for the lock then this function * returns -EINTR. * * This function is similar to (but not equivalent to) down_interruptible(). */ int __sched mutex_lock_interruptible(struct mutex *lock) { int ret; might_sleep(); ret = __mutex_fastpath_lock_retval (&lock->count, __mutex_lock_interruptible_slowpath); if (!ret) mutex_set_owner(lock); return ret; } EXPORT_SYMBOL(mutex_lock_interruptible); int __sched mutex_lock_killable(struct mutex *lock) { int ret; might_sleep(); ret = __mutex_fastpath_lock_retval (&lock->count, __mutex_lock_killable_slowpath); if (!ret) mutex_set_owner(lock); return ret; } EXPORT_SYMBOL(mutex_lock_killable); static __used noinline void __sched __mutex_lock_slowpath(atomic_t *lock_count) { struct mutex *lock = container_of(lock_count, struct mutex, count); __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, 0, _RET_IP_); } static noinline int __sched __mutex_lock_killable_slowpath(atomic_t *lock_count) { struct mutex *lock = container_of(lock_count, struct mutex, count); return __mutex_lock_common(lock, TASK_KILLABLE, 0, _RET_IP_); } static noinline int __sched __mutex_lock_interruptible_slowpath(atomic_t *lock_count) { struct mutex *lock = container_of(lock_count, struct mutex, count); return __mutex_lock_common(lock, TASK_INTERRUPTIBLE, 0, _RET_IP_); } #endif /* * Spinlock based trylock, we take the spinlock and check whether we * can get the lock: */ static inline int __mutex_trylock_slowpath(atomic_t *lock_count) { struct mutex *lock = container_of(lock_count, struct mutex, count); unsigned long flags; int prev; spin_lock_mutex(&lock->wait_lock, flags); prev = atomic_xchg(&lock->count, -1); if (likely(prev == 1)) { mutex_set_owner(lock); mutex_acquire(&lock->dep_map, 0, 1, _RET_IP_); } /* Set it back to 0 if there are no waiters: */ if (likely(list_empty(&lock->wait_list))) atomic_set(&lock->count, 0); spin_unlock_mutex(&lock->wait_lock, flags); return prev == 1; } /*** * mutex_trylock - try acquire the mutex, without waiting * @lock: the mutex to be acquired * * Try to acquire the mutex atomically. Returns 1 if the mutex * has been acquired successfully, and 0 on contention. * * NOTE: this function follows the spin_trylock() convention, so * it is negated to the down_trylock() return values! Be careful * about this when converting semaphore users to mutexes. * * This function must not be used in interrupt context. The * mutex must be released by the same task that acquired it. */ int __sched mutex_trylock(struct mutex *lock) { int ret; ret = __mutex_fastpath_trylock(&lock->count, __mutex_trylock_slowpath); if (ret) mutex_set_owner(lock); return ret; } EXPORT_SYMBOL(mutex_trylock); /** * atomic_dec_and_mutex_lock - return holding mutex if we dec to 0 * @cnt: the atomic which we are to dec * @lock: the mutex to return holding if we dec to 0 * * return true and hold lock if we dec to 0, return false otherwise */ int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock) { /* dec if we can't possibly hit 0 */ if (atomic_add_unless(cnt, -1, 1)) return 0; /* we might hit 0, so take the lock */ mutex_lock(lock); if (!atomic_dec_and_test(cnt)) { /* when we actually did the dec, we didn't hit 0 */ mutex_unlock(lock); return 0; } /* we hit 0, and we hold the lock */ return 1; } EXPORT_SYMBOL(atomic_dec_and_mutex_lock);