jbd2: 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.

Reported-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
index 6e28000..78c2992 100644 (file)
--- a/fs/jbd2/commit.c
+++ b/fs/jbd2/commit.c
@@ -760,8 +760,13 @@ wait_for_iobuf:
                    required. */
                JBUFFER_TRACE(jh, "file as BJ_Forget");
                jbd2_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
+                * jbd2_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);