Re: [PATCH] fs/direct-io.c: avoid workqueue allocation race

[Dave Chinner <david@fromorbit.com>]

[ 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@google.com>
> > > > 
> > > > 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@fromorbit.com