[ Sorry, my responses are limited at the moment because I took a
chunk out of a fingertip a couple of days ago and I can only do
about half an hour before my hand and arm start to cramp from the
weird positions and motions 3 finger typing results in.... ]
On Tue, Mar 10, 2020 at 09:27:58AM -0700, Darrick J. Wong wrote:
> On Sun, Mar 08, 2020 at 06:24:24PM -0700, Eric Biggers wrote:
> > On Mon, Mar 09, 2020 at 10:12:53AM +1100, Dave Chinner wrote:
> > > On Sat, Mar 07, 2020 at 09:52:21PM -0800, Eric Biggers wrote:
> > > > From: Eric Biggers <ebiggers@xxxxxxxxxx>
> > > >
> > > > When a thread loses the workqueue allocation race in
> > > > sb_init_dio_done_wq(), lockdep reports that the call to
> > > > destroy_workqueue() can deadlock waiting for work to complete. This is
> > > > a false positive since the workqueue is empty. But we shouldn't simply
> > > > skip the lockdep check for empty workqueues for everyone.
> > >
> > > Why not? If the wq is empty, it can't deadlock, so this is a problem
> > > with the workqueue lockdep annotations, not a problem with code that
> > > is destroying an empty workqueue.
> >
> > Skipping the lockdep check when flushing an empty workqueue would reduce the
> > ability of lockdep to detect deadlocks when flushing that workqueue. I.e., it
> > could cause lots of false negatives, since there are many cases where workqueues
> > are *usually* empty when flushed/destroyed but it's still possible that they are
> > nonempty.
> >
> > >
> > > > Just avoid this issue by using a mutex to serialize the workqueue
> > > > allocation. We still keep the preliminary check for ->s_dio_done_wq, so
> > > > this doesn't affect direct I/O performance.
> > > >
> > > > Also fix the preliminary check for ->s_dio_done_wq to use READ_ONCE(),
> > > > since it's a data race. (That part wasn't actually found by syzbot yet,
> > > > but it could be detected by KCSAN in the future.)
> > > >
> > > > Note: the lockdep false positive could alternatively be fixed by
> > > > introducing a new function like "destroy_unused_workqueue()" to the
> > > > workqueue API as previously suggested. But I think it makes sense to
> > > > avoid the double allocation anyway.
> > >
> > > Fix the infrastructure, don't work around it be placing constraints
> > > on how the callers can use the infrastructure to work around
> > > problems internal to the infrastructure.
> >
> > Well, it's also preferable not to make our debugging tools less effective to
> > support people doing weird things that they shouldn't really be doing anyway.
> >
> > (BTW, we need READ_ONCE() on ->sb_init_dio_done_wq anyway to properly annotate
> > the data race. That could be split into a separate patch though.)
> >
> > Another idea that came up is to make each workqueue_struct track whether work
> > has been queued on it or not yet, and make flush_workqueue() skip the lockdep
> > check if the workqueue has always been empty. (That could still cause lockdep
> > false negatives, but not as many as if we checked if the workqueue is
> > *currently* empty.) Would you prefer that solution? Adding more overhead to
> > workqueues would be undesirable though, so I think it would have to be
> > conditional on CONFIG_LOCKDEP, like (untested):
>
> I can't speak for Dave, but if the problem here really is that lockdep's
> modelling of flush_workqueue()'s behavior could be improved to eliminate
> false reports, then this seems reasonable to me...
Yeah, that's what I've been trying to say. IT seems much more
reasonable to fix it for everyone once with a few lines of code than
have to re-write every caller that might trip over this. e.g. think
of all the failure teardown paths that destroy workqueues without
having used them...
So, yeah, this seems like a much better approach....
> > diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> > index 301db4406bc37..72222c09bcaeb 100644
> > --- a/kernel/workqueue.c
> > +++ b/kernel/workqueue.c
> > @@ -263,6 +263,7 @@ struct workqueue_struct {
> > char *lock_name;
> > struct lock_class_key key;
> > struct lockdep_map lockdep_map;
> > + bool used;
> > #endif
> > char name[WQ_NAME_LEN]; /* I: workqueue name */
> >
> > @@ -1404,6 +1405,9 @@ static void __queue_work(int cpu, struct workqueue_struct *wq,
> > lockdep_assert_irqs_disabled();
> >
> > debug_work_activate(work);
> > +#ifdef CONFIG_LOCKDEP
> > + WRITE_ONCE(wq->used, true);
> > +#endif
....with an appropriate comment to explain why this code is needed.
Cheers,
Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
