bdi: Fix oops in wb_workfn()

commit b8b784958eccbf8f51ebeee65282ca3fd59ea391 upstream.

Syzbot has reported that it can hit a NULL pointer dereference in
wb_workfn() due to wb->bdi->dev being NULL. This indicates that
wb_workfn() was called for an already unregistered bdi which should not
happen as wb_shutdown() called from bdi_unregister() should make sure
all pending writeback works are completed before bdi is unregistered.
Except that wb_workfn() itself can requeue the work with:

	mod_delayed_work(bdi_wq, &wb->dwork, 0);

and if this happens while wb_shutdown() is waiting in:

	flush_delayed_work(&wb->dwork);

the dwork can get executed after wb_shutdown() has finished and
bdi_unregister() has cleared wb->bdi->dev.

Make wb_workfn() use wakeup_wb() for requeueing the work which takes all
the necessary precautions against racing with bdi unregistration.

CC: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
CC: Tejun Heo <tj@kernel.org>
Fixes: 839a8e8660
Reported-by: syzbot <syzbot+9873874c735f2892e7e9@syzkaller.appspotmail.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
[bwh: Backported to 3.16:
 - Use bdi_wakeup_thread() instead of wb_wakeup()
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

fs/fs-writeback.c

@@ -1072,7 +1072,7 @@ void bdi_writeback_workfn(struct work_struct *work)
 	}
 
 	if (!list_empty(&bdi->work_list))
-		mod_delayed_work(bdi_wq, &wb->dwork, 0);
+		bdi_wakeup_thread(bdi);
 	else if (wb_has_dirty_io(wb) && dirty_writeback_interval)
 		bdi_wakeup_thread_delayed(bdi);