From: Tejun Heo <tj@kernel.org>
To: Stephen Boyd <sboyd@codeaurora.org>
Cc: linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
Ben Dooks <ben-linux@fluff.org>
Subject: Re: [PATCH 1/2] workqueue: Catch more locking problems with flush_work()
Date: Fri, 20 Apr 2012 10:35:29 -0700 [thread overview]
Message-ID: <20120420173529.GD32324@google.com> (raw)
In-Reply-To: <4F905521.9020901@codeaurora.org>
Hello,
On Thu, Apr 19, 2012 at 11:10:41AM -0700, Stephen Boyd wrote:
> On 04/19/12 08:28, Tejun Heo wrote:
> > On Wed, Apr 18, 2012 at 08:25:57PM -0700, Stephen Boyd wrote:
> >> @@ -2513,8 +2513,11 @@ bool flush_work(struct work_struct *work)
> >> wait_for_completion(&barr.done);
> >> destroy_work_on_stack(&barr.work);
> >> return true;
> >> - } else
> >> + } else {
> >> + lock_map_acquire(&work->lockdep_map);
> >> + lock_map_release(&work->lockdep_map);
> >> return false;
> > We don't have this annotation when start_flush_work() succeeds either,
> > right? IOW, would lockdep trigger when an actual deadlock happens?
>
> I believe it does although I haven't tested it.
How does it do that? While wq->lockdep_map would be able to detect
some of the chaining, the read acquire paths probably would miss some
other. In general, wq->lockdep_map is used to express dependencies
regarding workqueue flushing (and the self flushing) and it would
probably be better to express work item dependencies explicitly using
work->lockdep_map even if it becomes redundant through wq->lockdep_map
sometimes.
> > If not, why not add the acquire/release() before flush_work() does
> > anything?
>
> I was worried about causing false positive lockdep warnings in the case
> that start_flush_work() succeeds and returns true. In that case, lockdep
> is told about the cwq lockdep map:
>
> static bool start_flush_work(struct work_struct *work, struct wq_barrier *barr,
> bool wait_executing)
> {
>
> .....
>
> if (cwq->wq->saved_max_active == 1 || cwq->wq->flags & WQ_RESCUER)
> lock_map_acquire(&cwq->wq->lockdep_map);
> else
> lock_map_acquire_read(&cwq->wq->lockdep_map);
>
>
> and so if we acquired the work->lockdep_map before the
> cwq->wq->lockdep_map we would get a warning about ABBA between these two
> lockdep maps. At least that is what I'm lead to believe when I look at
> what process_one_work() is doing. Please correct me if I'm wrong.
All that's necessary is acquiring and releasing work->lockdep_map.
There's no need to nest start_flush_work() inside it. Without
nesting, there's nothing to worry about ABBA lockdeps.
Thanks.
--
tejun
next prev parent reply other threads:[~2012-04-20 17:35 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-04-19 3:25 [PATCH 1/2] workqueue: Catch more locking problems with flush_work() Stephen Boyd
2012-04-19 3:25 ` [PATCH 2/2] ks8851: Fix mutex deadlock in ks8851_net_stop() Stephen Boyd
2012-04-21 19:34 ` David Miller
2012-04-19 8:10 ` [PATCH 1/2] workqueue: Catch more locking problems with flush_work() Yong Zhang
2012-04-19 18:36 ` Stephen Boyd
2012-04-20 5:26 ` Yong Zhang
2012-04-20 6:01 ` Yong Zhang
2012-04-20 6:26 ` Stephen Boyd
2012-04-20 7:18 ` Yong Zhang
2012-04-20 8:18 ` Stephen Boyd
2012-04-20 8:32 ` Yong Zhang
2012-04-21 0:32 ` Yong Zhang
2012-04-19 15:28 ` Tejun Heo
2012-04-19 18:10 ` Stephen Boyd
2012-04-20 17:35 ` Tejun Heo [this message]
2012-04-20 23:15 ` Stephen Boyd
2012-04-21 0:28 ` [PATCHv2] " Stephen Boyd
2012-04-21 0:34 ` Yong Zhang
2012-04-23 18:07 ` Tejun Heo
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