From patchwork Tue Jan 15 12:33:59 2013 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Maarten Lankhorst X-Patchwork-Id: 1977291 Return-Path: X-Original-To: patchwork-linux-media@patchwork.kernel.org Delivered-To: patchwork-process-083081@patchwork2.kernel.org Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by patchwork2.kernel.org (Postfix) with ESMTP id 14DECDF264 for ; Tue, 15 Jan 2013 12:35:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756519Ab3AOMe3 (ORCPT ); Tue, 15 Jan 2013 07:34:29 -0500 Received: from mail-wg0-f44.google.com ([74.125.82.44]:45540 "EHLO mail-wg0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756504Ab3AOMeZ (ORCPT ); Tue, 15 Jan 2013 07:34:25 -0500 Received: by mail-wg0-f44.google.com with SMTP id dr12so23980wgb.23 for ; Tue, 15 Jan 2013 04:34:23 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=x-received:from:to:cc:subject:date:message-id:x-mailer:in-reply-to :references; bh=QiB41jj0QyMDNRwpOEJCXDILpk+59/jxCHmdJdK6+vc=; b=FqCQUG9Q6bD1yL+mLnQcBCQtfe/UYQ2/SOHhsDFnEYe4PK4PRYiGW8oGYqD+JsJxrG evywE4GCDG0dkkEInxzYSHZKikgIhsqm6NyhZtWvIA0SBMrXv+NVLHnYEUY398dEBAFt 8HntLSsCbH8eQAa4LO4tgUunl+7ntSSxRXd5V/S7fztlDqw1UBmR3F8tHeC2A7Ptdz0y 3raivgsCrVe1cydvN7lywp/ciF6Wct/tL1rY0z9T8bDikeQS+0OuZrIFqgL8SztWYHmu sRJf2b2uFOh8JT3z4dIr8rzhwst3JXK7Ol71rxrPsKZ8DrRv409Gr11IzcwfZN0KIjuu E93Q== X-Received: by 10.180.109.195 with SMTP id hu3mr3365579wib.31.1358253263581; Tue, 15 Jan 2013 04:34:23 -0800 (PST) Received: from localhost (5ED48CEF.cm-7-5c.dynamic.ziggo.nl. [94.212.140.239]) by mx.google.com with ESMTPS id p2sm3593488wic.7.2013.01.15.04.34.21 (version=TLSv1 cipher=RC4-SHA bits=128/128); Tue, 15 Jan 2013 04:34:22 -0800 (PST) Received: by localhost (sSMTP sendmail emulation); Tue, 15 Jan 2013 13:34:20 +0100 From: Maarten Lankhorst To: dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, linux-media@vger.kernel.org, linaro-mm-sig@lists.linaro.org Cc: Maarten Lankhorst Subject: [PATCH 2/7] mutex: add support for reservation style locks Date: Tue, 15 Jan 2013 13:33:59 +0100 Message-Id: <1358253244-11453-3-git-send-email-maarten.lankhorst@canonical.com> X-Mailer: git-send-email 1.8.0.3 In-Reply-To: <1358253244-11453-1-git-send-email-maarten.lankhorst@canonical.com> References: <1358253244-11453-1-git-send-email-maarten.lankhorst@canonical.com> Sender: linux-media-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-media@vger.kernel.org makes it easier to port ttm over.. Signed-off-by: Maarten Lankhorst Signed-off-by: Maarten Lankhorst --- include/linux/mutex.h | 86 +++++++++++++- kernel/mutex.c | 317 +++++++++++++++++++++++++++++++++++++++++++++++--- 2 files changed, 387 insertions(+), 16 deletions(-) diff --git a/include/linux/mutex.h b/include/linux/mutex.h index 9121595..602c247 100644 --- a/include/linux/mutex.h +++ b/include/linux/mutex.h @@ -62,6 +62,11 @@ struct mutex { #endif }; +struct ticket_mutex { + struct mutex base; + atomic_long_t reservation_id; +}; + /* * This is the control structure for tasks blocked on mutex, * which resides on the blocked task's kernel stack: @@ -109,12 +114,24 @@ static inline void mutex_destroy(struct mutex *lock) {} __DEBUG_MUTEX_INITIALIZER(lockname) \ __DEP_MAP_MUTEX_INITIALIZER(lockname) } +#define __TICKET_MUTEX_INITIALIZER(lockname) \ + { .base = __MUTEX_INITIALIZER(lockname) \ + , .reservation_id = ATOMIC_LONG_INIT(0) } + #define DEFINE_MUTEX(mutexname) \ struct mutex mutexname = __MUTEX_INITIALIZER(mutexname) extern void __mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key); +static inline void __ticket_mutex_init(struct ticket_mutex *lock, + const char *name, + struct lock_class_key *key) +{ + __mutex_init(&lock->base, name, key); + atomic_long_set(&lock->reservation_id, 0); +} + /** * mutex_is_locked - is the mutex locked * @lock: the mutex to be queried @@ -133,26 +150,91 @@ static inline int mutex_is_locked(struct mutex *lock) #ifdef CONFIG_DEBUG_LOCK_ALLOC extern void mutex_lock_nested(struct mutex *lock, unsigned int subclass); extern void _mutex_lock_nest_lock(struct mutex *lock, struct lockdep_map *nest_lock); + extern int __must_check mutex_lock_interruptible_nested(struct mutex *lock, unsigned int subclass); extern int __must_check mutex_lock_killable_nested(struct mutex *lock, unsigned int subclass); +extern int __must_check _mutex_reserve_lock(struct ticket_mutex *lock, + struct lockdep_map *nest_lock, + unsigned long reservation_id); + +extern int __must_check _mutex_reserve_lock_interruptible(struct ticket_mutex *, + struct lockdep_map *nest_lock, + unsigned long reservation_id); + +extern void _mutex_reserve_lock_slow(struct ticket_mutex *lock, + struct lockdep_map *nest_lock, + unsigned long reservation_id); + +extern int __must_check _mutex_reserve_lock_intr_slow(struct ticket_mutex *, + struct lockdep_map *nest_lock, + unsigned long reservation_id); + #define mutex_lock(lock) mutex_lock_nested(lock, 0) #define mutex_lock_interruptible(lock) mutex_lock_interruptible_nested(lock, 0) #define mutex_lock_killable(lock) mutex_lock_killable_nested(lock, 0) #define mutex_lock_nest_lock(lock, nest_lock) \ do { \ - typecheck(struct lockdep_map *, &(nest_lock)->dep_map); \ + typecheck(struct lockdep_map *, &(nest_lock)->dep_map); \ _mutex_lock_nest_lock(lock, &(nest_lock)->dep_map); \ } while (0) +#define mutex_reserve_lock(lock, nest_lock, reservation_id) \ +({ \ + typecheck(struct lockdep_map *, &(nest_lock)->dep_map); \ + _mutex_reserve_lock(lock, &(nest_lock)->dep_map, reservation_id); \ +}) + +#define mutex_reserve_lock_interruptible(lock, nest_lock, reservation_id) \ +({ \ + typecheck(struct lockdep_map *, &(nest_lock)->dep_map); \ + _mutex_reserve_lock_interruptible(lock, &(nest_lock)->dep_map, \ + reservation_id); \ +}) + +#define mutex_reserve_lock_slow(lock, nest_lock, reservation_id) \ +do { \ + typecheck(struct lockdep_map *, &(nest_lock)->dep_map); \ + _mutex_reserve_lock_slow(lock, &(nest_lock)->dep_map, reservation_id); \ +} while (0) + +#define mutex_reserve_lock_intr_slow(lock, nest_lock, reservation_id) \ +({ \ + typecheck(struct lockdep_map *, &(nest_lock)->dep_map); \ + _mutex_reserve_lock_intr_slow(lock, &(nest_lock)->dep_map, \ + reservation_id); \ +}) + #else extern void mutex_lock(struct mutex *lock); extern int __must_check mutex_lock_interruptible(struct mutex *lock); extern int __must_check mutex_lock_killable(struct mutex *lock); +extern int __must_check _mutex_reserve_lock(struct ticket_mutex *lock, + unsigned long reservation_id); +extern int __must_check _mutex_reserve_lock_interruptible(struct ticket_mutex *, + unsigned long reservation_id); + +extern void _mutex_reserve_lock_slow(struct ticket_mutex *lock, + unsigned long reservation_id); +extern int __must_check _mutex_reserve_lock_intr_slow(struct ticket_mutex *, + unsigned long reservation_id); + +#define mutex_reserve_lock(lock, nest_lock, reservation_id) \ + _mutex_reserve_lock(lock, reservation_id) + +#define mutex_reserve_lock_interruptible(lock, nest_lock, reservation_id) \ + _mutex_reserve_lock_interruptible(lock, reservation_id) + +#define mutex_reserve_lock_slow(lock, nest_lock, reservation_id) \ + _mutex_reserve_lock_slow(lock, reservation_id) + +#define mutex_reserve_lock_intr_slow(lock, nest_lock, reservation_id) \ + _mutex_reserve_lock_intr_slow(lock, reservation_id) + # define mutex_lock_nested(lock, subclass) mutex_lock(lock) # define mutex_lock_interruptible_nested(lock, subclass) mutex_lock_interruptible(lock) # define mutex_lock_killable_nested(lock, subclass) mutex_lock_killable(lock) @@ -167,6 +249,8 @@ extern int __must_check mutex_lock_killable(struct mutex *lock); */ extern int mutex_trylock(struct mutex *lock); extern void mutex_unlock(struct mutex *lock); +extern void mutex_unreserve_unlock(struct ticket_mutex *lock); + extern int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock); #ifndef CONFIG_HAVE_ARCH_MUTEX_CPU_RELAX diff --git a/kernel/mutex.c b/kernel/mutex.c index a307cc9..8282729 100644 --- a/kernel/mutex.c +++ b/kernel/mutex.c @@ -126,16 +126,119 @@ void __sched mutex_unlock(struct mutex *lock) EXPORT_SYMBOL(mutex_unlock); +/** + * mutex_unreserve_unlock - release the mutex + * @lock: the mutex to be released + * + * Unlock a mutex that has been locked by this task previously + * with _mutex_reserve_lock*. + * + * This function must not be used in interrupt context. Unlocking + * of a not locked mutex is not allowed. + */ +void __sched mutex_unreserve_unlock(struct ticket_mutex *lock) +{ + /* + * mark mutex as no longer part of a reservation, next + * locker can set this again + */ + atomic_long_set(&lock->reservation_id, 0); + + /* + * 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->base); +#endif + __mutex_fastpath_unlock(&lock->base.count, __mutex_unlock_slowpath); +} +EXPORT_SYMBOL(mutex_unreserve_unlock); + +static inline int __sched +__mutex_lock_check_reserve(struct mutex *lock, unsigned long reservation_id) +{ + struct ticket_mutex *m = container_of(lock, struct ticket_mutex, base); + unsigned long cur_id; + + cur_id = atomic_long_read(&m->reservation_id); + if (!cur_id) + return 0; + + if (unlikely(reservation_id == cur_id)) + return -EDEADLK; + + if (unlikely(reservation_id - cur_id <= LONG_MAX)) + return -EAGAIN; + + return 0; +} + +/* + * after acquiring lock with fastpath or when we lost out in contested + * slowpath, set reservation_id and wake up any waiters so they can recheck. + */ +static __always_inline void +mutex_set_reservation_fastpath(struct ticket_mutex *lock, + unsigned long reservation_id, bool check_res) +{ + unsigned long flags; + struct mutex_waiter *cur; + + if (check_res || config_enabled(CONFIG_DEBUG_LOCK_ALLOC)) { + unsigned long cur_id; + + cur_id = atomic_long_xchg(&lock->reservation_id, + reservation_id); +#ifdef CONFIG_DEBUG_LOCK_ALLOC + if (check_res) + DEBUG_LOCKS_WARN_ON(cur_id && + cur_id != reservation_id); + else + DEBUG_LOCKS_WARN_ON(cur_id); + lockdep_assert_held(&lock->base); +#endif + + if (unlikely(cur_id == reservation_id)) + return; + } else + atomic_long_set(&lock->reservation_id, reservation_id); + + /* + * 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 reservation_id + */ + 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 inline int __sched __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, - struct lockdep_map *nest_lock, unsigned long ip) + struct lockdep_map *nest_lock, unsigned long ip, + unsigned long reservation_id, bool res_slow) { 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); @@ -162,6 +265,12 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, for (;;) { struct task_struct *owner; + if (!__builtin_constant_p(reservation_id) && !res_slow) { + ret = __mutex_lock_check_reserve(lock, reservation_id); + if (ret) + goto err_nowait; + } + /* * If there's an owner, wait for it to either * release the lock or go to sleep. @@ -172,6 +281,13 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, if (atomic_cmpxchg(&lock->count, 1, 0) == 1) { lock_acquired(&lock->dep_map, ip); + if (res_slow) { + struct ticket_mutex *m; + m = container_of(lock, struct ticket_mutex, base); + + mutex_set_reservation_fastpath(m, reservation_id, false); + } + mutex_set_owner(lock); preempt_enable(); return 0; @@ -227,15 +343,16 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, * 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); + ret = -EINTR; + goto err; + } - debug_mutex_free_waiter(&waiter); - preempt_enable(); - return -EINTR; + if (!__builtin_constant_p(reservation_id) && !res_slow) { + ret = __mutex_lock_check_reserve(lock, reservation_id); + if (ret) + goto err; } + __set_task_state(task, state); /* didn't get the lock, go to sleep: */ @@ -250,6 +367,28 @@ done: mutex_remove_waiter(lock, &waiter, current_thread_info()); mutex_set_owner(lock); + if (!__builtin_constant_p(reservation_id)) { + struct ticket_mutex *m; + struct mutex_waiter *cur; + /* + * this should get optimized out for the common case, + * and is only important for _mutex_reserve_lock + */ + + m = container_of(lock, struct ticket_mutex, base); + atomic_long_set(&m->reservation_id, reservation_id); + + /* + * give any possible sleeping processes the chance to wake up, + * so they can recheck if they have to back off from + * reservations + */ + list_for_each_entry(cur, &lock->wait_list, list) { + debug_mutex_wake_waiter(lock, cur); + wake_up_process(cur->task); + } + } + /* set it to 0 if there are no waiters left: */ if (likely(list_empty(&lock->wait_list))) atomic_set(&lock->count, 0); @@ -260,6 +399,19 @@ done: preempt_enable(); return 0; + +err: + mutex_remove_waiter(lock, &waiter, task_thread_info(task)); + spin_unlock_mutex(&lock->wait_lock, flags); + debug_mutex_free_waiter(&waiter); + +#ifdef CONFIG_MUTEX_SPIN_ON_OWNER +err_nowait: +#endif + mutex_release(&lock->dep_map, 1, ip); + + preempt_enable(); + return ret; } #ifdef CONFIG_DEBUG_LOCK_ALLOC @@ -267,7 +419,8 @@ void __sched mutex_lock_nested(struct mutex *lock, unsigned int subclass) { might_sleep(); - __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, subclass, NULL, _RET_IP_); + __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, + subclass, NULL, _RET_IP_, 0, 0); } EXPORT_SYMBOL_GPL(mutex_lock_nested); @@ -276,7 +429,8 @@ void __sched _mutex_lock_nest_lock(struct mutex *lock, struct lockdep_map *nest) { might_sleep(); - __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, 0, nest, _RET_IP_); + __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, + 0, nest, _RET_IP_, 0, 0); } EXPORT_SYMBOL_GPL(_mutex_lock_nest_lock); @@ -285,7 +439,8 @@ int __sched mutex_lock_killable_nested(struct mutex *lock, unsigned int subclass) { might_sleep(); - return __mutex_lock_common(lock, TASK_KILLABLE, subclass, NULL, _RET_IP_); + return __mutex_lock_common(lock, TASK_KILLABLE, + subclass, NULL, _RET_IP_, 0, 0); } EXPORT_SYMBOL_GPL(mutex_lock_killable_nested); @@ -294,10 +449,63 @@ mutex_lock_interruptible_nested(struct mutex *lock, unsigned int subclass) { might_sleep(); return __mutex_lock_common(lock, TASK_INTERRUPTIBLE, - subclass, NULL, _RET_IP_); + subclass, NULL, _RET_IP_, 0, 0); } EXPORT_SYMBOL_GPL(mutex_lock_interruptible_nested); + +int __sched +_mutex_reserve_lock(struct ticket_mutex *lock, struct lockdep_map *nest, + unsigned long reservation_id) +{ + DEBUG_LOCKS_WARN_ON(!reservation_id); + + might_sleep(); + return __mutex_lock_common(&lock->base, TASK_UNINTERRUPTIBLE, + 0, nest, _RET_IP_, reservation_id, 0); +} +EXPORT_SYMBOL_GPL(_mutex_reserve_lock); + + +int __sched +_mutex_reserve_lock_interruptible(struct ticket_mutex *lock, + struct lockdep_map *nest, + unsigned long reservation_id) +{ + DEBUG_LOCKS_WARN_ON(!reservation_id); + + might_sleep(); + return __mutex_lock_common(&lock->base, TASK_INTERRUPTIBLE, + 0, nest, _RET_IP_, reservation_id, 0); +} +EXPORT_SYMBOL_GPL(_mutex_reserve_lock_interruptible); + +void __sched +_mutex_reserve_lock_slow(struct ticket_mutex *lock, struct lockdep_map *nest, + unsigned long reservation_id) +{ + DEBUG_LOCKS_WARN_ON(!reservation_id); + + might_sleep(); + __mutex_lock_common(&lock->base, TASK_UNINTERRUPTIBLE, 0, + nest, _RET_IP_, reservation_id, 1); +} +EXPORT_SYMBOL_GPL(_mutex_reserve_lock_slow); + +int __sched +_mutex_reserve_lock_intr_slow(struct ticket_mutex *lock, + struct lockdep_map *nest, + unsigned long reservation_id) +{ + DEBUG_LOCKS_WARN_ON(!reservation_id); + + might_sleep(); + return __mutex_lock_common(&lock->base, TASK_INTERRUPTIBLE, 0, + nest, _RET_IP_, reservation_id, 1); +} +EXPORT_SYMBOL_GPL(_mutex_reserve_lock_intr_slow); + + #endif /* @@ -400,7 +608,8 @@ __mutex_lock_slowpath(atomic_t *lock_count) { struct mutex *lock = container_of(lock_count, struct mutex, count); - __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, 0, NULL, _RET_IP_); + __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, 0, + NULL, _RET_IP_, 0, 0); } static noinline int __sched @@ -408,7 +617,8 @@ __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, NULL, _RET_IP_); + return __mutex_lock_common(lock, TASK_KILLABLE, 0, + NULL, _RET_IP_, 0, 0); } static noinline int __sched @@ -416,8 +626,28 @@ __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, NULL, _RET_IP_); + return __mutex_lock_common(lock, TASK_INTERRUPTIBLE, 0, + NULL, _RET_IP_, 0, 0); +} + +static noinline int __sched +__mutex_lock_reserve_slowpath(atomic_t *lock_count, void *rid) +{ + struct mutex *lock = container_of(lock_count, struct mutex, count); + + return __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, 0, + NULL, _RET_IP_, (unsigned long)rid, 0); +} + +static noinline int __sched +__mutex_lock_interruptible_reserve_slowpath(atomic_t *lock_count, void *rid) +{ + struct mutex *lock = container_of(lock_count, struct mutex, count); + + return __mutex_lock_common(lock, TASK_INTERRUPTIBLE, 0, + NULL, _RET_IP_, (unsigned long)rid, 0); } + #endif /* @@ -473,6 +703,63 @@ int __sched mutex_trylock(struct mutex *lock) } EXPORT_SYMBOL(mutex_trylock); +#ifndef CONFIG_DEBUG_LOCK_ALLOC +int __sched +_mutex_reserve_lock(struct ticket_mutex *lock, unsigned long rid) +{ + int ret; + + might_sleep(); + + ret = __mutex_fastpath_lock_retval_arg(&lock->base.count, (void *)rid, + __mutex_lock_reserve_slowpath); + + if (!ret) { + mutex_set_reservation_fastpath(lock, rid, true); + mutex_set_owner(&lock->base); + } + return ret; +} +EXPORT_SYMBOL(_mutex_reserve_lock); + +int __sched +_mutex_reserve_lock_interruptible(struct ticket_mutex *lock, unsigned long rid) +{ + int ret; + + might_sleep(); + + ret = __mutex_fastpath_lock_retval_arg(&lock->base.count, (void *)rid, + __mutex_lock_interruptible_reserve_slowpath); + + if (!ret) { + mutex_set_reservation_fastpath(lock, rid, true); + mutex_set_owner(&lock->base); + } + return ret; +} +EXPORT_SYMBOL(_mutex_reserve_lock_interruptible); + +void __sched +_mutex_reserve_lock_slow(struct ticket_mutex *lock, unsigned long rid) +{ + might_sleep(); + __mutex_lock_common(&lock->base, TASK_UNINTERRUPTIBLE, + 0, NULL, _RET_IP_, rid, 1); +} +EXPORT_SYMBOL(_mutex_reserve_lock_slow); + +int __sched +_mutex_reserve_lock_intr_slow(struct ticket_mutex *lock, unsigned long rid) +{ + might_sleep(); + return __mutex_lock_common(&lock->base, TASK_INTERRUPTIBLE, + 0, NULL, _RET_IP_, rid, 1); +} +EXPORT_SYMBOL(_mutex_reserve_lock_intr_slow); + +#endif + /** * atomic_dec_and_mutex_lock - return holding mutex if we dec to 0 * @cnt: the atomic which we are to dec