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

From: Yong Zhang <[email-masked]>
Date: Fri, 20 Apr 2012 14:01:01 +0800

On Fri, Apr 20, 2012 at 01:26:33PM +0800, Yong Zhang wrote:
> On Thu, Apr 19, 2012 at 11:36:32AM -0700, Stephen Boyd wrote:
> > Does looking at the second patch help? Basically schedule_work() can run
> > the callback right between the time the mutex is acquired and
> > flush_work() is called:
> > 
> > CPU0                        CPU1
> > 
> > <irq>
> >   schedule_work()           mutex_lock(&mutex)
> > <irq return>
> >     my_work()               flush_work() 
> >       mutex_lock(&mutex)    
> >       <deadlock>
> 
> Get you point. It is a problem. But your patch could introduece false
> positive since when flush_work() is called that very work may finish
> running already.
> 
> So I think we need the lock_map_acquire()/lock_map_release() only when
> the work is under processing, no?

But start_flush_work() has tried take care of this issue except it
doesn't add work->lockdep_map into the chain.

So does below patch help?

Thanks,
Yong

---
From: Yong Zhang <[email-masked]>
Date: Fri, 20 Apr 2012 13:44:16 +0800
Subject: [PATCH] workqueue:lockdep: make flush_work notice deadlock

Connet the lock chain by aquiring work->lockdep_map when
the tobe-flush work is running.

Signed-off-by: Yong Zhang <[email-masked]>
Reported-by: Stephen Boyd <[email-masked]>
---
 kernel/workqueue.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index bc867e8..c096b05 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2461,6 +2461,8 @@ static bool start_flush_work(struct work_struct *work, struct wq_barrier *barr,
 		lock_map_acquire(&cwq->wq->lockdep_map);
 	else
 		lock_map_acquire_read(&cwq->wq->lockdep_map);
+	lock_map_acquire(&work->lockdep_map);
+	lock_map_release(&work->lockdep_map);
 	lock_map_release(&cwq->wq->lockdep_map);
 
 	return true;
-- 
1.7.5.4