ext4: fix deadlock during page writeback

ext4: fix deadlock during page writeback

Orabug: 25308117

[ Upstream commit 646caa9c8e196880b41cd3e3d33a2ebc752bdb85 ]

Commit 06bd3c36a733 (ext4: fix data exposure after a crash) uncovered a
deadlock in ext4_writepages() which was previously much harder to hit.
After this commit xfstest generic/130 reproduces the deadlock on small
filesystems.

The problem happens when ext4_do_update_inode() sets LARGE_FILE feature
and marks current inode handle as synchronous. That subsequently results
in ext4_journal_stop() called from ext4_writepages() to block waiting for
transaction commit while still holding page locks, reference to io_end,
and some prepared bio in mpd structure each of which can possibly block
transaction commit from completing and thus results in deadlock.

Fix the problem by releasing page locks, io_end reference, and
submitting prepared bio before calling ext4_journal_stop().

[ Changed to defer the call to ext4_journal_stop() only if the handle
  is synchronous.  --tytso ]

Reported-and-tested-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
CC: stable@vger.kernel.org
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
(cherry picked from commit 906d6f4d9cdc8509c505f29f6146ec627fef2f06)
Signed-off-by: Dhaval Giani <dhaval.giani@oracle.com>