jbd: Fix forever sleeping process in do_get_write_access()

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.

CC: stable@kernel.org
Reported-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: Jan Kara <jack@suse.cz>

diff --git a/fs/jbd/commit.c b/fs/jbd/commit.c
index 69b18045946330a5a35c54226094ca0dc657a903..f486ff6977813bdc3f9ec009c57b75941f7a26d7 100644 (file)
--- 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);