Simplify semaphore implementation
By removing the negative values of 'count' and relying on the wait_list to
indicate whether we have any waiters, we can simplify the implementation
by removing the protection against an unlikely race condition. Thanks to
David Howells for his suggestions.
Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
diff --git a/include/linux/semaphore.h b/include/linux/semaphore.h
index a107aeb..a7125da 100644
--- a/include/linux/semaphore.h
+++ b/include/linux/semaphore.h
@@ -15,15 +15,12 @@
/*
* The spinlock controls access to the other members of the semaphore.
- * 'count' is decremented by every task which calls down*() and incremented
- * by every call to up(). Thus, if it is positive, it indicates how many
- * more tasks may acquire the lock. If it is negative, it indicates how
- * many tasks are waiting for the lock. Tasks waiting for the lock are
- * kept on the wait_list.
+ * 'count' represents how many more tasks can acquire this semaphore.
+ * Tasks waiting for the lock are kept on the wait_list.
*/
struct semaphore {
spinlock_t lock;
- int count;
+ unsigned int count;
struct list_head wait_list;
};
diff --git a/kernel/semaphore.c b/kernel/semaphore.c
index 5a12a85..bef977b 100644
--- a/kernel/semaphore.c
+++ b/kernel/semaphore.c
@@ -18,18 +18,8 @@
* 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).
+ * 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);
@@ -43,7 +33,9 @@
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);
}
@@ -55,7 +47,9 @@
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);
@@ -69,7 +63,9 @@
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);
@@ -111,7 +107,9 @@
int result = 0;
spin_lock_irqsave(&sem->lock, flags);
- if (unlikely(sem->count-- <= 0))
+ if (likely(sem->count > 0))
+ sem->count--;
+ else
result = __down_timeout(sem, jiffies);
spin_unlock_irqrestore(&sem->lock, flags);
@@ -124,7 +122,7 @@
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);
@@ -141,22 +139,6 @@
};
/*
- * 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. Likewise the
* 'timeout' parameter for the cases without timeouts.
@@ -164,7 +146,6 @@
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;
@@ -184,28 +165,16 @@
timeout = schedule_timeout(timeout);
spin_lock_irq(&sem->lock);
if (waiter.up)
- goto woken;
+ return 0;
}
timed_out:
list_del(&waiter.list);
- result = -ETIME;
- goto woken;
+ 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)
@@ -230,8 +199,9 @@
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);
}
