From 714493cd5468f42ca3c4f730a9c17c203abd5059 Mon Sep 17 00:00:00 2001 From: Matthew Wilcox Date: Fri, 11 Apr 2008 15:23:52 -0400 Subject: Improve semaphore documentation Move documentation from semaphore.h to semaphore.c as requested by Andrew Morton. Also reformat to kernel-doc style and add some more notes about the implementation. Signed-off-by: Matthew Wilcox --- kernel/semaphore.c | 79 ++++++++++++++++++++++++++++++++++++++++++++++-------- 1 file changed, 68 insertions(+), 11 deletions(-) (limited to 'kernel/semaphore.c') diff --git a/kernel/semaphore.c b/kernel/semaphore.c index bef977b1696..5c2942e768c 100644 --- a/kernel/semaphore.c +++ b/kernel/semaphore.c @@ -3,6 +3,26 @@ * Author: Matthew Wilcox * * Distributed under the terms of the GNU GPL, version 2 + * + * This file implements counting semaphores. + * A counting semaphore may be acquired 'n' times before sleeping. + * See mutex.c for single-acquisition sleeping locks which enforce + * rules which allow code to be debugged more easily. + */ + +/* + * Some notes on the implementation: + * + * The spinlock controls access to the other members of the semaphore. + * down_trylock() and up() can be called from interrupt context, so we + * have to disable interrupts when taking the lock. It turns out various + * parts of the kernel expect to be able to use down() on a semaphore in + * interrupt context when they know it will succeed, so we have to use + * irqsave variants for down(), down_interruptible() and down_killable() + * too. + * + * The ->count variable represents how many more tasks can acquire this + * semaphore. If it's zero, there may be tasks waiting on the wait_list. */ #include @@ -12,22 +32,23 @@ #include #include -/* - * Some notes on the implementation: - * - * down_trylock() and up() can be called from interrupt context. - * So we have to disable interrupts when taking the lock. - * - * The ->count variable defines how many more tasks can acquire the - * semaphore. If it's zero, there may be tasks waiting on the list. - */ - static noinline void __down(struct semaphore *sem); static noinline int __down_interruptible(struct semaphore *sem); static noinline int __down_killable(struct semaphore *sem); static noinline int __down_timeout(struct semaphore *sem, long jiffies); static noinline void __up(struct semaphore *sem); +/** + * down - acquire the semaphore + * @sem: the semaphore to be acquired + * + * Acquires the semaphore. If no more tasks are allowed to acquire the + * semaphore, calling this function will put the task to sleep until the + * semaphore is released. + * + * Use of this function is deprecated, please use down_interruptible() or + * down_killable() instead. + */ void down(struct semaphore *sem) { unsigned long flags; @@ -41,6 +62,15 @@ void down(struct semaphore *sem) } EXPORT_SYMBOL(down); +/** + * down_interruptible - acquire the semaphore unless interrupted + * @sem: the semaphore to be acquired + * + * Attempts to acquire the semaphore. If no more tasks are allowed to + * acquire the semaphore, calling this function will put the task to sleep. + * If the sleep is interrupted by a signal, this function will return -EINTR. + * If the semaphore is successfully acquired, this function returns 0. + */ int down_interruptible(struct semaphore *sem) { unsigned long flags; @@ -57,6 +87,16 @@ int down_interruptible(struct semaphore *sem) } EXPORT_SYMBOL(down_interruptible); +/** + * down_killable - acquire the semaphore unless killed + * @sem: the semaphore to be acquired + * + * Attempts to acquire the semaphore. If no more tasks are allowed to + * acquire the semaphore, calling this function will put the task to sleep. + * If the sleep is interrupted by a fatal signal, this function will return + * -EINTR. If the semaphore is successfully acquired, this function returns + * 0. + */ int down_killable(struct semaphore *sem) { unsigned long flags; @@ -78,7 +118,7 @@ EXPORT_SYMBOL(down_killable); * @sem: the semaphore to be acquired * * Try to acquire the semaphore atomically. Returns 0 if the mutex has - * been acquired successfully and 1 if it is contended. + * been acquired successfully or 1 if it it cannot be acquired. * * NOTE: This return value is inverted from both spin_trylock and * mutex_trylock! Be careful about this when converting code. @@ -101,6 +141,16 @@ int down_trylock(struct semaphore *sem) } EXPORT_SYMBOL(down_trylock); +/** + * down_timeout - acquire the semaphore within a specified time + * @sem: the semaphore to be acquired + * @jiffies: how long to wait before failing + * + * Attempts to acquire the semaphore. If no more tasks are allowed to + * acquire the semaphore, calling this function will put the task to sleep. + * If the semaphore is not released within the specified number of jiffies, + * this function returns -ETIME. It returns 0 if the semaphore was acquired. + */ int down_timeout(struct semaphore *sem, long jiffies) { unsigned long flags; @@ -117,6 +167,13 @@ int down_timeout(struct semaphore *sem, long jiffies) } EXPORT_SYMBOL(down_timeout); +/** + * up - release the semaphore + * @sem: the semaphore to release + * + * Release the semaphore. Unlike mutexes, up() may be called from any + * context and even by tasks which have never called down(). + */ void up(struct semaphore *sem) { unsigned long flags; -- cgit v1.2.3