* Author: Matthew Wilcox <willy@linux.intel.com>
*
* 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 <linux/compiler.h>
#include <linux/sched.h>
#include <linux/semaphore.h>
#include <linux/spinlock.h>
-
-/*
- * 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, if positive, defines how many more tasks can
- * acquire the semaphore. If negative, it represents how many tasks are
- * waiting on the semaphore (*). If zero, no tasks are waiting, and no more
- * tasks can acquire the semaphore.
- *
- * (*) Except for the window between one task calling up() and the task
- * sleeping in a __down_common() waking up. In order to avoid a third task
- * coming in and stealing the second task's wakeup, we leave the ->count
- * negative. If we have a more complex situation, the ->count may become
- * zero or negative (eg a semaphore with count = 2, three tasks attempt to
- * acquire it, one sleeps, two finish and call up(), the second task to call
- * up() notices that the list is empty and just increments count).
- */
+#include <linux/ftrace.h>
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;
spin_lock_irqsave(&sem->lock, flags);
- if (unlikely(sem->count-- <= 0))
+ if (likely(sem->count > 0))
+ sem->count--;
+ else
__down(sem);
spin_unlock_irqrestore(&sem->lock, flags);
}
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;
int result = 0;
spin_lock_irqsave(&sem->lock, flags);
- if (unlikely(sem->count-- <= 0))
+ if (likely(sem->count > 0))
+ sem->count--;
+ else
result = __down_interruptible(sem);
spin_unlock_irqrestore(&sem->lock, flags);
}
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;
int result = 0;
spin_lock_irqsave(&sem->lock, flags);
- if (unlikely(sem->count-- <= 0))
+ if (likely(sem->count > 0))
+ sem->count--;
+ else
result = __down_killable(sem);
spin_unlock_irqrestore(&sem->lock, flags);
* @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.
}
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;
+ int result = 0;
+
+ spin_lock_irqsave(&sem->lock, flags);
+ if (likely(sem->count > 0))
+ sem->count--;
+ else
+ result = __down_timeout(sem, jiffies);
+ spin_unlock_irqrestore(&sem->lock, flags);
+
+ return result;
+}
+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;
spin_lock_irqsave(&sem->lock, flags);
- if (likely(sem->count >= 0))
+ if (likely(list_empty(&sem->wait_list)))
sem->count++;
else
__up(sem);
};
/*
- * Wake up a process waiting on a semaphore. We need to call this from both
- * __up and __down_common as it's possible to race a task into the semaphore
- * if it comes in at just the right time between two tasks calling up() and
- * a third task waking up. This function assumes the wait_list is already
- * checked for being non-empty.
- */
-static noinline void __sched __up_down_common(struct semaphore *sem)
-{
- struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list,
- struct semaphore_waiter, list);
- list_del(&waiter->list);
- waiter->up = 1;
- wake_up_process(waiter->task);
-}
-
-/*
- * Because this function is inlined, the 'state' parameter will be constant,
- * and thus optimised away by the compiler.
+ * Because this function is inlined, the 'state' parameter will be
+ * constant, and thus optimised away by the compiler. Likewise the
+ * 'timeout' parameter for the cases without timeouts.
*/
-static inline int __sched __down_common(struct semaphore *sem, long state)
+static inline int __sched __down_common(struct semaphore *sem, long state,
+ long timeout)
{
- int result = 0;
struct task_struct *task = current;
struct semaphore_waiter waiter;
waiter.up = 0;
for (;;) {
- if (state == TASK_INTERRUPTIBLE && signal_pending(task))
- goto interrupted;
- if (state == TASK_KILLABLE && fatal_signal_pending(task))
+ if (signal_pending_state(state, task))
goto interrupted;
+ if (timeout <= 0)
+ goto timed_out;
__set_task_state(task, state);
spin_unlock_irq(&sem->lock);
- schedule();
+ timeout = schedule_timeout(timeout);
spin_lock_irq(&sem->lock);
if (waiter.up)
- goto woken;
+ return 0;
}
+ timed_out:
+ list_del(&waiter.list);
+ return -ETIME;
+
interrupted:
list_del(&waiter.list);
- result = -EINTR;
- woken:
- /*
- * Account for the process which woke us up. For the case where
- * we're interrupted, we need to increment the count on our own
- * behalf. I don't believe we can hit the case where the
- * sem->count hits zero, *and* there's a second task sleeping,
- * but it doesn't hurt, that's not a commonly exercised path and
- * it's not a performance path either.
- */
- if (unlikely((++sem->count >= 0) && !list_empty(&sem->wait_list)))
- __up_down_common(sem);
- return result;
+ return -EINTR;
}
static noinline void __sched __down(struct semaphore *sem)
{
- __down_common(sem, TASK_UNINTERRUPTIBLE);
+ __down_common(sem, TASK_UNINTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
}
static noinline int __sched __down_interruptible(struct semaphore *sem)
{
- return __down_common(sem, TASK_INTERRUPTIBLE);
+ return __down_common(sem, TASK_INTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
}
static noinline int __sched __down_killable(struct semaphore *sem)
{
- return __down_common(sem, TASK_KILLABLE);
+ return __down_common(sem, TASK_KILLABLE, MAX_SCHEDULE_TIMEOUT);
+}
+
+static noinline int __sched __down_timeout(struct semaphore *sem, long jiffies)
+{
+ return __down_common(sem, TASK_UNINTERRUPTIBLE, jiffies);
}
static noinline void __sched __up(struct semaphore *sem)
{
- if (unlikely(list_empty(&sem->wait_list)))
- sem->count++;
- else
- __up_down_common(sem);
+ struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list,
+ struct semaphore_waiter, list);
+ list_del(&waiter->list);
+ waiter->up = 1;
+ wake_up_process(waiter->task);
}
git://ftp.safe.ca / safe / jmp / linux-2.6 / blobdiff