linux-kernel - [PATCH v2 09/13] rwsem: skip initial trylock in rwsem_down_write

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Message-Id: <1363344869-15732-10-git-send-email-walken@google.com>
Date:	Fri, 15 Mar 2013 03:54:25 -0700
From:	Michel Lespinasse <walken@...gle.com>
To:	Alex Shi <alex.shi@...el.com>, Ingo Molnar <mingo@...nel.org>,
	David Howells <dhowells@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Yuanhan Liu <yuanhan.liu@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Dave Chinner <david@...morbit.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: [PATCH v2 09/13] 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>

---
 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);
 	}
-- 
1.8.1.3
--
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