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... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
