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Date: Tue, 7 May 2013 06:45:57 -0700 From: Michel Lespinasse <walken@...gle.com> To: Linus Torvalds <torvalds@...ux-foundation.org>, David Howells <dhowells@...hat.com>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>, Davidlohr Bueso <davidlohr.bueso@...com>, Peter Hurley <peter@...leysoftware.com> Cc: Alex Shi <alex.shi@...el.com>, Yuanhan Liu <yuanhan.liu@...ux.intel.com>, Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org Subject: [PATCH v3 09/14] 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@...gle.com> Reviewed-by: Peter Hurley <peter@...leysoftware.com> Acked-by: Davidlohr Bueso <davidlohr.bueso@...com> Acked-by: Rik van Riel <riel@...hat.com> --- lib/rwsem.c | 17 +++++++++-------- 1 file 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); } -- 1.8.2.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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