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From: Tejun Heo <tj@kernel.org>
To: Bart Van Assche <bvanassche@acm.org>
Cc: linux-kernel@vger.kernel.org,
	Johannes Berg <johannes.berg@intel.com>,
	Christoph Hellwig <hch@lst.de>, Sagi Grimberg <sagi@grimberg.me>,
	tytso@mit.edu
Subject: Re: [PATCH 3/3] kernel/workqueue: Suppress a false positive lockdep complaint
Date: Thu, 25 Oct 2018 08:37:34 -0700	[thread overview]
Message-ID: <20181025153734.GH4182586@devbig004.ftw2.facebook.com> (raw)
In-Reply-To: <20181025153657.GG4182586@devbig004.ftw2.facebook.com>

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

  reply	other threads:[~2018-10-25 15:37 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-25 15:05 [PATCH 0/3] Suppress false positives triggered by workqueue lockdep annotations Bart Van Assche
2018-10-25 15:05 ` [PATCH 1/3] kernel/workqueue: Remove lockdep annotation from __flush_work() Bart Van Assche
2018-10-25 15:31   ` Johannes Berg
2018-10-25 15:57     ` Johannes Berg
2018-10-25 16:01     ` Bart Van Assche
2018-10-25 15:05 ` [PATCH 2/3] kernel/workqueue: Surround work execution with shared lock annotations Bart Van Assche
2018-10-25 16:53   ` Johannes Berg
2018-10-25 17:22     ` Bart Van Assche
2018-10-25 19:17       ` Johannes Berg
2018-10-25 15:05 ` [PATCH 3/3] kernel/workqueue: Suppress a false positive lockdep complaint Bart Van Assche
2018-10-25 15:34   ` Johannes Berg
2018-10-25 15:55     ` Bart Van Assche
2018-10-25 19:59       ` Johannes Berg
2018-10-25 20:21         ` Theodore Y. Ts'o
2018-10-25 20:26           ` Johannes Berg
2018-10-25 15:36   ` Tejun Heo
2018-10-25 15:37     ` Tejun Heo [this message]
2018-10-25 20:13     ` Johannes Berg
2018-10-25 15:40   ` Theodore Y. Ts'o
2018-10-25 17:02   ` Johannes Berg
2018-10-25 17:11     ` Bart Van Assche
2018-10-25 19:51       ` Johannes Berg
2018-10-25 20:39         ` Bart Van Assche
2018-10-25 20:47           ` Johannes Berg
2018-10-25 15:27 ` [PATCH 0/3] Suppress false positives triggered by workqueue lockdep annotations Johannes Berg
2018-10-25 15:47   ` Bart Van Assche

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