From 9b0fc9c09f1b262b7fe697eba6b05095d78850e5 Mon Sep 17 00:00:00 2001 From: Michel Lespinasse Date: Tue, 7 May 2013 06:45:57 -0700 Subject: [PATCH] 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 Reviewed-by: Peter Hurley Acked-by: Davidlohr Bueso Acked-by: Rik van Riel Signed-off-by: Linus Torvalds --- lib/rwsem.c | 17 +++++++++-------- 1 file changed, 9 insertions(+), 8 deletions(-) diff --git a/lib/rwsem.c b/lib/rwsem.c index edf3d9c..0d50e46 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); } -- 2.7.4