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

[Darrick J. Wong]

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...

--D

> 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
>  
>  	/* if draining, only works from the same workqueue are allowed */
>  	if (unlikely(wq->flags & __WQ_DRAINING) &&
> @@ -2772,8 +2776,12 @@ void flush_workqueue(struct workqueue_struct *wq)
>  	if (WARN_ON(!wq_online))
>  		return;
>  
> -	lock_map_acquire(&wq->lockdep_map);
> -	lock_map_release(&wq->lockdep_map);
> +#ifdef CONFIG_LOCKDEP
> +	if (READ_ONCE(wq->used)) {
> +		lock_map_acquire(&wq->lockdep_map);
> +		lock_map_release(&wq->lockdep_map);
> +	}
> +#endif
>  
>  	mutex_lock(&wq->mutex);