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

[Cong Wang]

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.

Thanks!
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