On 5/3/18 3:55 PM, Dave Chinner wrote: > On Thu, May 03, 2018 at 06:26:26PM +0200, Jan Kara wrote: >> 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@xxxxxxxxxxxxxxxxxxx> >> CC: Tejun Heo <tj@xxxxxxxxxx> >> Fixes: 839a8e8660b6777e7fe4e80af1a048aebe2b5977 >> Reported-by: syzbot <syzbot+9873874c735f2892e7e9@xxxxxxxxxxxxxxxxxxxxxxxxx> >> Signed-off-by: Jan Kara <jack@xxxxxxx> >> --- >> fs/fs-writeback.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c >> index 47d7c151fcba..471d863958bc 100644 >> --- a/fs/fs-writeback.c >> +++ b/fs/fs-writeback.c >> @@ -1961,7 +1961,7 @@ void wb_workfn(struct work_struct *work) >> } >> >> if (!list_empty(&wb->work_list)) >> - mod_delayed_work(bdi_wq, &wb->dwork, 0); >> + wb_wakeup(wb); >> else if (wb_has_dirty_io(wb) && dirty_writeback_interval) >> wb_wakeup_delayed(wb); > > Yup, looks fine - I can't see any more of these open coded wakeup, > either, so we should be good here. > > Reviewed-by: Dave Chinner <dchinner@xxxxxxxxxx> > > As an aside, why is half the wb infrastructure in fs/fs-writeback.c > and the other half in mm/backing-dev.c? it seems pretty random as to > what is where e.g. wb_wakeup() and wb_wakeup_delayed() are almost > identical, but are in completely different files... That's always bothered me too, it's due for a cleanup and bringing it all into one location. -- Jens Axboe
