From: Chris Mason <mason@suse.com>

jbd needs to wait for any io to complete on the buffer before changing the
end_io function.  Using set_buffer_locked means that it can change the
end_io function while the page is in the middle of writeback, and the
writeback bit on the page will never get cleared.

Since we set the buffer dirty earlier on, if the page was previously dirty,
pdflush or memory pressure might trigger a writepage call, which will race
with jbd's set_buffer_locked.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>