Re: [PATCH UPDATED] workqueue: relax lockdep annotation on flush_work()

[Peter Zijlstra <peterz@infradead.org>]

On Mon, 2011-01-03 at 15:17 +0100, Tejun Heo wrote:

> @@ -2384,8 +2384,18 @@ static bool start_flush_work(struct work_struct *work, struct wq_barrier *barr,
>  	insert_wq_barrier(cwq, barr, work, worker);
>  	spin_unlock_irq(&gcwq->lock);
>  
> -	lock_map_acquire(&cwq->wq->lockdep_map);
> +	/*
> +	 * If @max_active is 1 or rescuer is in use, flushing another work
> +	 * item on the same workqueue may lead to deadlock.  Make sure the
> +	 * flusher is not running on the same workqueue by verifying write
> +	 * access.
> +	 */
> +	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);
>  	lock_map_release(&cwq->wq->lockdep_map);
> +
>  	return true;
>  already_gone:
>  	spin_unlock_irq(&gcwq->lock);

Ah, but this violates the rule that you must always use the most strict
constraints. Code doesn't know if it will run in a rescue thread or not,
hence it must assume it does.