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Subject[044/146] jbd: Fix forever sleeping process in do_get_write_access()
2.6.38-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Jan Kara <jack@suse.cz>

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@taobao.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
fs/jbd/commit.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)

--- a/fs/jbd/commit.c
+++ b/fs/jbd/commit.c
@@ -722,8 +722,13 @@ 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);



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