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Date:	Wed, 24 Jun 2015 17:16:06 -0700
From:	Dave Hansen <>
Subject: [RFCv2][PATCH 3/7] fs: fsnotify: replace memory barrier in __sb_end_write() with RCU

From: Dave Hansen <>

If I sit in a loop and do write()s to small tmpfs files,
__sb_end_write() is third-hottest kernel function due to its

__sb_end_write() uses the barrier to avoid races with freeze_super()
and its calls to sb_wait_write().  But, now that freeze_super() is
calling synchronize_rcu() before each sb_wait_write() call, we can
use that to our advantage.

The synchronize_rcu() ensures that all __sb_end_write() will see
freeze_super()'s updates to s_writers.counter.  That, in turn,
guarantees that __sb_end_write() will try to wake up any subsequent
call by freeze_super() to sb_wait_write().

Cc: Jan Kara <>
Cc: Alexander Viro <>
Cc: Paul E. McKenney <>
Cc: Tim Chen <>
Cc: Andi Kleen <>
Signed-off-by: Dave Hansen <>

 b/fs/super.c |   17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff -puN fs/super.c~selectively-do-barriers-in-__sb_end_write fs/super.c
--- a/fs/super.c~selectively-do-barriers-in-__sb_end_write	2015-06-24 17:14:35.315142611 -0700
+++ b/fs/super.c	2015-06-24 17:14:35.318142745 -0700
@@ -1146,14 +1146,23 @@ out:
 void __sb_end_write(struct super_block *sb, int level)
+	rcu_read_lock();
-	 * Make sure s_writers are updated before we wake up waiters in
-	 * freeze_super().
+	 * We are racing here with freeze_super()'s calls to
+	 * sb_wait_write().  We want to ensure that we call
+	 * wake_up() whenever one of those calls _might_ be
+	 * in sb_wait_write().
+	 *
+	 * Since freeze_super() does a synchronize_rcu() before
+	 * each of its sb_wait_write() calls, it can guarantee
+	 * that it sees our update to s_writers.counter as well
+	 * as that we see its update to s_writers.frozen.
-	smp_mb();
-	if (waitqueue_active(&sb->s_writers.wait))
+	if (unlikely(sb->s_writers.frozen >= level))
+	rcu_read_unlock();
 	rwsem_release(&sb->s_writers.lock_map[level-1], 1, _RET_IP_);
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