linux-kernel - [PATCH 2/3] percpu-rwsem: don't use percpu_rw_semaphore->rw

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Date:	Mon, 29 Jun 2015 01:56:42 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	paulmck@...ux.vnet.ibm.com, tj@...nel.org, mingo@...hat.com,
	der.herr@...r.at, dave@...olabs.net, riel@...hat.com,
	viro@...IV.linux.org.uk, torvalds@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH 2/3] percpu-rwsem: don't use percpu_rw_semaphore->rw_sem to
	exclude writers

percpu_down_write() does down_write() to exclude both the readers and
other writers. We can rely on rcu_sync_enter() in exclusive mode and
take ->rw_sem right before wait_event().

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
 kernel/locking/percpu-rwsem.c |    3 +--
 1 files changed, 1 insertions(+), 2 deletions(-)

diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
index 014d2f4..609c13b 100644
--- a/kernel/locking/percpu-rwsem.c
+++ b/kernel/locking/percpu-rwsem.c
@@ -139,8 +139,6 @@ static bool readers_active_check(struct percpu_rw_semaphore *sem)
 
 void percpu_down_write(struct percpu_rw_semaphore *sem)
 {
-	down_write(&sem->rw_sem);
-
 	/* Notify readers to take the slow path. */
 	rcu_sync_enter(&sem->rss);
 
@@ -158,6 +156,7 @@ void percpu_down_write(struct percpu_rw_semaphore *sem)
 	 * therefore will wait for them.
 	 */
 
+	down_write(&sem->rw_sem);
 	/* Wait for all now active readers to complete. */
 	wait_event(sem->writer, readers_active_check(sem));
 }
-- 
1.5.5.1

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