Re: [Patch] workqueue: move lockdep annotations up to destroy_workqueue()

[Cong Wang]

Cong Wang wrote:
> Cong Wang wrote:
> > Tejun Heo wrote:
> > > Hello,
> > >
> > > On 04/01/2010 01:28 PM, Cong Wang wrote:
> > > > > Hmmm... can you please try to see whether this circular locking
> > > > > warning involving wq->lockdep_map is reproducible w/ the bonding
> > > > > locking fixed?  I still can't see where wq -> cpu_add_remove_lock
> > > > > dependency is created.
> > > > I thought this is obvious.
> > > >
> > > > Here it is:
> > > >
> > > > void destroy_workqueue(struct workqueue_struct *wq)
> > > > {
> > > >         const struct cpumask *cpu_map = wq_cpu_map(wq);
> > > >         int cpu;
> > > >
> > > >         cpu_maps_update_begin();        <----------------- Hold
> > > > cpu_add_remove_lock here
> > > >         spin_lock(&workqueue_lock);
> > > >         list_del(&wq->list);
> > > >         spin_unlock(&workqueue_lock);
> > > >
> > > >         for_each_cpu(cpu, cpu_map)
> > > >                 cleanup_workqueue_thread(per_cpu_ptr(wq->cpu_wq, 
> > > > cpu)); <------ See below
> > > >         cpu_maps_update_done();        <----------------- Release
> > > > cpu_add_remove_lock here
> > > >
> > > > ...
> > > > static void cleanup_workqueue_thread(struct cpu_workqueue_struct *cwq)
> > > > {
> > > >         /*
> > > >          * Our caller is either destroy_workqueue() or CPU_POST_DEAD,
> > > >          * cpu_add_remove_lock protects cwq->thread.
> > > >          */
> > > >         if (cwq->thread == NULL)
> > > >                 return;
> > > >
> > > >         lock_map_acquire(&cwq->wq->lockdep_map); <-------------- 
> > > > Lockdep
> > > > complains here.
> > > >         lock_map_release(&cwq->wq->lockdep_map);
> > > > ...
> > >
> > > Yeap, the above is cpu_add_remove_lock -> wq->lockdep_map dependency.
> > > I can see that but I'm failing to see where the dependency the other
> > > direction is created.
> >
> > Hmm, it looks like I misunderstand lock_map_acquire()? From the 
> > changelog,
> > I thought it was added to complain its caller is holding a lock when 
> > invoking
> > it, thus cpu_add_remove_lock is not an exception.
>
> Oh, I see, wq->lockdep_map is acquired again in run_workqueue(), so I 
> was wrong. :)
> I think you and Oleg are right, the lockdep warning is not irrelevant.

Oops, typo, I meant "is irrelevant." ;)

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