diff options
| author | Michel Lespinasse <walken@google.com> | 2013-05-07 06:45:55 -0700 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2013-05-07 07:20:16 -0700 |
| commit | 5ede972df1cd9294c82e9515949fd2103be81d7b (patch) | |
| tree | 2c6df4c16fbe4430ea2723f2aec284105aab08db | |
| parent | ed00f64346631dff035adfb9b0240daaa8b46c4e (diff) | |
rwsem: use cmpxchg for trying to steal write lock
Using rwsem_atomic_update to try stealing the write lock forced us to
undo the adjustment in the failure path. We can have simpler and faster
code by using cmpxchg instead.
Signed-off-by: Michel Lespinasse <walken@google.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
| -rw-r--r-- | lib/rwsem.c | 26 |
1 files changed, 6 insertions, 20 deletions
diff --git a/lib/rwsem.c b/lib/rwsem.c index 2360bf204098..64c2dc007be2 100644 --- a/lib/rwsem.c +++ b/lib/rwsem.c @@ -142,25 +142,6 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type) return sem; } -/* Try to get write sem, caller holds sem->wait_lock: */ -static int try_get_writer_sem(struct rw_semaphore *sem) -{ - long oldcount, adjustment; - - adjustment = RWSEM_ACTIVE_WRITE_BIAS; - if (list_is_singular(&sem->wait_list)) - adjustment -= RWSEM_WAITING_BIAS; - -try_again_write: - oldcount = rwsem_atomic_update(adjustment, sem) - adjustment; - if (!(oldcount & RWSEM_ACTIVE_MASK)) - return 1; - /* some one grabbed the sem already */ - if (rwsem_atomic_update(-adjustment, sem) & RWSEM_ACTIVE_MASK) - return 0; - goto try_again_write; -} - /* * wait for the read lock to be granted */ @@ -236,7 +217,12 @@ struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem) while (true) { set_task_state(tsk, TASK_UNINTERRUPTIBLE); - if (try_get_writer_sem(sem)) + /* Try acquiring the write lock. */ + count = RWSEM_ACTIVE_WRITE_BIAS; + if (!list_is_singular(&sem->wait_list)) + count += RWSEM_WAITING_BIAS; + if (cmpxchg(&sem->count, RWSEM_WAITING_BIAS, count) == + RWSEM_WAITING_BIAS) break; raw_spin_unlock_irq(&sem->wait_lock); |