Re: [PATCH 1/2] workqueue: Catch more locking problems with flush_work()

From: Stephen Boyd
Date: Thu Apr 19 2012 - 14:10:54 EST

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(&;
>> 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.

> 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)

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.

Sent by an employee of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.

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