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Commit da16922c authored by Michel Lespinasse's avatar Michel Lespinasse Committed by Linus Torvalds
Browse files

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: default avatarMichel Lespinasse <walken@google.com>
Reviewed-by: default avatarRik van Riel <riel@redhat.com>
Reviewed-by: default avatarPeter Hurley <peter@hurleysoftware.com>
Acked-by: default avatarDavidlohr Bueso <davidlohr.bueso@hp.com>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 1e78277c

lib/rwsem.c

+2 −20
Original line number Diff line number Diff line
@@ -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();

	}