linux-kernel - Re: net/bluetooth: workqueue destruction WARNING in hci_unregister

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Date:	Fri, 18 Mar 2016 16:52:31 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Jiri Slaby <jslaby@...e.cz>
Cc:	Dmitry Vyukov <dvyukov@...gle.com>,
	Marcel Holtmann <marcel@...tmann.org>,
	Gustavo Padovan <gustavo@...ovan.org>,
	Johan Hedberg <johan.hedberg@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	linux-bluetooth@...r.kernel.org, netdev <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	syzkaller <syzkaller@...glegroups.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Takashi Iwai <tiwai@...e.com>
Subject: Re: net/bluetooth: workqueue destruction WARNING in
 hci_unregister_dev

Hello, Jiri.

On Thu, Mar 17, 2016 at 01:00:13PM +0100, Jiri Slaby wrote:
> >> I have not done that yet, but today, I see:
> >> destroy_workqueue: name='req_hci0' pwq=ffff88002f590300
> >> wq->dfl_pwq=ffff88002f591e00 pwq->refcnt=2 pwq->nr_active=0 delayed_works:
> >>    pwq 12: cpus=0-1 node=0 flags=0x4 nice=-20 active=0/1
> >>      in-flight: 18568:wq_barrier_func
> > 
> > So, this means that there's flush_work() racing against workqueue
> > destruction, which can't be safe. :(
> 
> But I cannot trigger the WARN_ONs in the attached patch, so I am
> confused how this can happen :(. (While I am still seeing the destroy
> WARNINGs.)

So, no operations should be in progress when destroy_workqueue() is
called.  If somebody was flushing a work item, the flush call must
have returned before destroy_workqueue() was invoked, which doesn't
seem to be the case here.  Can you trigger BUG_ON() or sysrq-t when
the above triggers?  There must be a task which is flushing a work
item there and it shouldn't be difficult to pinpoint what's going on
from it.

Thanks.

-- 
tejun