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

[Peter Zijlstra <peterz@infradead.org>]

On Mon, 2011-01-03 at 16:00 +0100, Tejun Heo wrote:
> Hello,
> 
> On Mon, Jan 03, 2011 at 03:54:50PM +0100, Peter Zijlstra wrote:
> > 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.
> 
> Hmmm?  The code applies the most strict contraints.  If the workqueue
> has a rescuer, flushing another work from the workqueue will always
> trigger lockdep warning.  The rule is relaxed only for workqueues
> which aren't used for memory reclaiming && support parallel execution.

Ah, ok. I read it like: if the current thread is a rescue thread.