linux-kernel - [34-longterm 011/179] jbd: Fix forever sleeping process in do_get_write

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Message-ID: <1337048075-6132-12-git-send-email-paul.gortmaker@windriver.com>
Date:	Mon, 14 May 2012 22:11:47 -0400
From:	Paul Gortmaker <paul.gortmaker@...driver.com>
To:	<stable@...nel.org>, <linux-kernel@...r.kernel.org>
Subject: [34-longterm 011/179] jbd: Fix forever sleeping process in do_get_write_access()

From: Jan Kara <jack@...e.cz>

                   -------------------
    This is a commit scheduled for the next v2.6.34 longterm release.
    http://git.kernel.org/?p=linux/kernel/git/paulg/longterm-queue-2.6.34.git
    If you see a problem with using this for longterm, please comment.
                   -------------------

commit 2842bb20eed2e25cde5114298edc62c8883a1d9a upstream.

In do_get_write_access() we wait on BH_Unshadow bit for buffer to get
from shadow state. The waking code in journal_commit_transaction() has
a bug because it does not issue a memory barrier after the buffer is moved
from the shadow state and before wake_up_bit() is called. Thus a waitqueue
check can happen before the buffer is actually moved from the shadow state
and waiting process may never be woken. Fix the problem by issuing proper
barrier.

Reported-by: Tao Ma <boyu.mt@...bao.com>
Signed-off-by: Jan Kara <jack@...e.cz>
Signed-off-by: Paul Gortmaker <paul.gortmaker@...driver.com>
---
 fs/jbd/commit.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/fs/jbd/commit.c b/fs/jbd/commit.c
index ecb44c9..1df9270 100644
--- a/fs/jbd/commit.c
+++ b/fs/jbd/commit.c
@@ -745,8 +745,13 @@ wait_for_iobuf:
                    required. */
 		JBUFFER_TRACE(jh, "file as BJ_Forget");
 		journal_file_buffer(jh, commit_transaction, BJ_Forget);
-		/* Wake up any transactions which were waiting for this
-		   IO to complete */
+		/*
+		 * Wake up any transactions which were waiting for this
+		 * IO to complete. The barrier must be here so that changes
+		 * by journal_file_buffer() take effect before wake_up_bit()
+		 * does the waitqueue check.
+		 */
+		smp_mb();
 		wake_up_bit(&bh->b_state, BH_Unshadow);
 		JBUFFER_TRACE(jh, "brelse shadowed buffer");
 		__brelse(bh);
-- 
1.7.9.6

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