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rwsem: simplify rwsem_down_read_failed 

When trying to acquire a read lock, the RWSEM_ACTIVE_READ_BIAS
adjustment doesn't cause other readers to block, so we never have to
worry about waking them back after canceling this adjustment in
rwsem_down_read_failed().

We also never want to steal the lock in rwsem_down_read_failed(), so we
don't have to grab the wait_lock either.

Signed-off-by: Michel Lespinasse <walken@google.com>
Reviewed-by: Rik van Riel <riel@redhat.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 06:45:52 -07:00 committed by Linus Torvalds
parent 1e78277ccb
commit da16922cc0
1 changed files with 2 additions and 20 deletions
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22
lib/rwsem.c
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@ -182,7 +182,6 @@ try_again_write:
*/
struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
{
enum rwsem_waiter_type type = RWSEM_WAITING_FOR_READ;
signed long adjustment = -RWSEM_ACTIVE_READ_BIAS;
struct rwsem_waiter waiter;
struct task_struct *tsk = current;
@ -190,7 +189,7 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
/* set up my own style of waitqueue */
waiter.task = tsk;
waiter.type = type;
waiter.type = RWSEM_WAITING_FOR_READ;
get_task_struct(tsk);
raw_spin_lock_irq(&sem->wait_lock);
@ -201,17 +200,9 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
/* we're now waiting on the lock, but no longer actively locking */
count = rwsem_atomic_update(adjustment, sem);
/* If there are no active locks, wake the front queued process(es) up.
*
* Alternatively, if we're called from a failed down_write(), there
* were already threads queued before us and there are no active
* writers, the lock must be read owned; so we try to wake any read
* locks that were queued ahead of us. */
/* If there are no active locks, wake the front queued process(es). */
if (count == RWSEM_WAITING_BIAS)
sem = __rwsem_do_wake(sem, RWSEM_WAKE_NO_ACTIVE);
else if (count > RWSEM_WAITING_BIAS &&
adjustment == -RWSEM_ACTIVE_WRITE_BIAS)
sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);
raw_spin_unlock_irq(&sem->wait_lock);
@ -220,15 +211,6 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
set_task_state(tsk, TASK_UNINTERRUPTIBLE);
if (!waiter.task)
break;
raw_spin_lock_irq(&sem->wait_lock);
/* Try to get the writer sem, may steal from the head writer: */
if (type == RWSEM_WAITING_FOR_WRITE)
if (try_get_writer_sem(sem, &waiter)) {
raw_spin_unlock_irq(&sem->wait_lock);
return sem;
}
raw_spin_unlock_irq(&sem->wait_lock);
schedule();
}