diff options
| author | Michel Lespinasse <walken@google.com> | 2013-05-07 06:45:57 -0700 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2013-05-07 07:20:16 -0700 |
| commit | 9b0fc9c09f1b262b7fe697eba6b05095d78850e5 (patch) | |
| tree | 5fb25ae32c9aed77f604858a77a27cd4b37bbfc6 /lib | |
| parent | a7d2c573ae7fad1b2c877d1a1342fa5bb0d6478c (diff) | |
| download | lwn-9b0fc9c09f1b262b7fe697eba6b05095d78850e5.tar.gz lwn-9b0fc9c09f1b262b7fe697eba6b05095d78850e5.zip | |
rwsem: skip initial trylock in rwsem_down_write_failed
We can skip the initial trylock in rwsem_down_write_failed() if there
are known active lockers already, thus saving one likely-to-fail
cmpxchg.
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>
Diffstat (limited to 'lib')
| -rw-r--r-- | lib/rwsem.c | 17 |
1 files changed, 9 insertions, 8 deletions
diff --git a/lib/rwsem.c b/lib/rwsem.c index edf3d9ca670e..0d50e46d5b0c 100644 --- a/lib/rwsem.c +++ b/lib/rwsem.c @@ -216,14 +216,15 @@ struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem) /* wait until we successfully acquire the lock */ set_task_state(tsk, TASK_UNINTERRUPTIBLE); while (true) { - - /* 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) == + if (!(count & RWSEM_ACTIVE_MASK)) { + /* 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; + break; + } raw_spin_unlock_irq(&sem->wait_lock); @@ -231,7 +232,7 @@ struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem) do { schedule(); set_task_state(tsk, TASK_UNINTERRUPTIBLE); - } while (sem->count & RWSEM_ACTIVE_MASK); + } while ((count = sem->count) & RWSEM_ACTIVE_MASK); raw_spin_lock_irq(&sem->wait_lock); } |