Re: [PATCH 3/3] kernel/workqueue: Suppress a false positive lockdep complaint

[Tejun Heo]

On Thu, Oct 25, 2018 at 08:36:57AM -0700, Tejun Heo wrote:
> Hello, Bart.
> 
> On Thu, Oct 25, 2018 at 08:05:40AM -0700, Bart Van Assche wrote:
> > diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
> > index 60d673e15632..375ec764f148 100644
> > --- a/include/linux/workqueue.h
> > +++ b/include/linux/workqueue.h
> > @@ -344,6 +344,7 @@ enum {
> >  	__WQ_ORDERED		= 1 << 17, /* internal: workqueue is ordered */
> >  	__WQ_LEGACY		= 1 << 18, /* internal: create*_workqueue() */
> >  	__WQ_ORDERED_EXPLICIT	= 1 << 19, /* internal: alloc_ordered_workqueue() */
> > +	__WQ_HAS_BEEN_USED	= 1 << 20, /* internal: work has been queued */
> >  
> >  	WQ_MAX_ACTIVE		= 512,	  /* I like 512, better ideas? */
> >  	WQ_MAX_UNBOUND_PER_CPU	= 4,	  /* 4 * #cpus for unbound wq */
> > diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> > index fc9129d5909e..0ef275fe526c 100644
> > --- a/kernel/workqueue.c
> > +++ b/kernel/workqueue.c
> > @@ -1383,6 +1383,10 @@ static void __queue_work(int cpu, struct workqueue_struct *wq,
> >  	if (unlikely(wq->flags & __WQ_DRAINING) &&
> >  	    WARN_ON_ONCE(!is_chained_work(wq)))
> >  		return;
> > +
> > +	if (!(wq->flags & __WQ_HAS_BEEN_USED))
> > +		wq->flags |= __WQ_HAS_BEEN_USED;
> > +
> >  retry:
> >  	if (req_cpu == WORK_CPU_UNBOUND)
> >  		cpu = wq_select_unbound_cpu(raw_smp_processor_id());
> > @@ -2889,7 +2893,7 @@ static bool start_flush_work(struct work_struct *work, struct wq_barrier *barr,
> >  	 * workqueues the deadlock happens when the rescuer stalls, blocking
> >  	 * forward progress.
> >  	 */
> > -	if (!from_cancel &&
> > +	if (!from_cancel && (pwq->wq->flags & __WQ_HAS_BEEN_USED) &&
> >  	    (pwq->wq->saved_max_active == 1 || pwq->wq->rescuer)) {
> >  		lock_acquire_exclusive(&pwq->wq->lockdep_map, 0, 0, NULL,
> >  				       _THIS_IP_);
> 
> We likely wanna skip the whole drain instead of eliding lockdep
> annotation here.  Other than that, this patch looks fine to me but for
> the others, I think it'd be a better idea to listen to Johannes.  We
> wanna annotate the users for the exceptions rather than weakening the
> workqueue lockdep checks, especially because workqueue related
> deadlocks can be pretty difficult to trigger and root cause
> afterwards.

Ooh, also, please only do the HAS_BEEN_USED marking if LOCKDEP is
enabled.

Thanks.

-- 
tejun