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rwsem: simplify __rwsem_do_wake

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This is mostly for cleanup value:

- We don't need several gotos to handle the case where the first
  waiter is a writer. Two simple tests will do (and generate very
  similar code).

- In the remainder of the function, we know the first waiter is a reader,
  so we don't have to double check that. We can use do..while loops
  to iterate over the readers to wake (generates slightly better code).

Signed-off-by: Michel Lespinasse <walken@google.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Michel Lespinasse 2013-05-07 06:45:58 -07:00 committed by Linus Torvalds
parent 9b0fc9c09f
commit 8cf5322ce6
2 changed files with 19 additions and 30 deletions
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23
lib/rwsem-spinlock.c
View File

@ -70,26 +70,17 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
if (!wakewrite) {
if (waiter->type == RWSEM_WAITING_FOR_WRITE)
goto out;
goto dont_wake_writers;
}
/*
* as we support write lock stealing, we can't set sem->activity
* to -1 here to indicate we get the lock. Instead, we wake it up
* to let it go get it again.
*/
if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
wake_up_process(waiter->task);
if (wakewrite)
/* Wake up a writer. Note that we do not grant it the
* lock - it will have to acquire it when it runs. */
wake_up_process(waiter->task);
goto out;
}
/* grant an infinite number of read locks to the front of the queue */
dont_wake_writers:
woken = 0;
while (waiter->type == RWSEM_WAITING_FOR_READ) {
do {
struct list_head *next = waiter->list.next;
list_del(&waiter->list);
@ -99,10 +90,10 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
wake_up_process(tsk);
put_task_struct(tsk);
woken++;
if (list_empty(&sem->wait_list))
if (next == &sem->wait_list)
break;
waiter = list_entry(next, struct rwsem_waiter, list);
}
} while (waiter->type != RWSEM_WAITING_FOR_WRITE);
sem->activity += woken;

26
lib/rwsem.c
View File

@ -68,20 +68,17 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
signed long woken, loop, adjustment;
waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
if (waiter->type != RWSEM_WAITING_FOR_WRITE)
goto readers_only;
if (wake_type == RWSEM_WAKE_READ_OWNED)
/* Another active reader was observed, so wakeup is not
* likely to succeed. Save the atomic op.
*/
if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
if (wake_type != RWSEM_WAKE_READ_OWNED)
/* Wake writer at the front of the queue, but do not
* grant it the lock yet as we want other writers
* to be able to steal it. Readers, on the other hand,
* will block as they will notice the queued writer.
*/
wake_up_process(waiter->task);
goto out;
}
/* Wake up the writing waiter and let the task grab the sem: */
wake_up_process(waiter->task);
goto out;
readers_only:
/* If we come here from up_xxxx(), another thread might have reached
* rwsem_down_failed_common() before we acquired the spinlock and
* woken up a waiter, making it now active. We prefer to check for
@ -125,7 +122,8 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
rwsem_atomic_add(adjustment, sem);
next = sem->wait_list.next;
for (loop = woken; loop > 0; loop--) {
loop = woken;
do {
waiter = list_entry(next, struct rwsem_waiter, list);
next = waiter->list.next;
tsk = waiter->task;
@ -133,7 +131,7 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
waiter->task = NULL;
wake_up_process(tsk);
put_task_struct(tsk);
}
} while (--loop);
sem->wait_list.next = next;
next->prev = &sem->wait_list;