On Wed, Oct 03, 2012 at 09:05:31AM +0530, Srivatsa S. Bhat wrote:
> On 10/03/2012 03:47 AM, Jiri Kosina wrote:
> > On Wed, 3 Oct 2012, Srivatsa S. Bhat wrote:
> >
> >> I don't see how this circular locking dependency can occur.. If you are
> >> using SLUB,
> >> kmem_cache_destroy() releases slab_mutex before it calls rcu_barrier(). If
> >> you are
> >> using SLAB, kmem_cache_destroy() wraps its whole operation inside
> >> get/put_online_cpus(),
> >> which means, it cannot run concurrently with a hotplug operation such as
> >> cpu_up(). So, I'm
> >> rather puzzled at this lockdep splat..
> >
> > I am using SLAB here.
> >
> > The scenario I think is very well possible:
> >
> >
> > CPU 0 CPU 1
> > kmem_cache_destroy()
>
> What about the get_online_cpus() right here at CPU0 before
> calling mutex_lock(slab_mutex)? How can the cpu_up() proceed
> on CPU1?? I still don't get it... :(
>
> (kmem_cache_destroy() uses get/put_online_cpus() around acquiring
> and releasing slab_mutex).
The problem is that there is a CPU-hotplug notifier for slab, which
establishes hotplug->slab. Then having kmem_cache_destroy() call
rcu_barrier() under the lock establishes slab->hotplug, which results
in deadlock. Jiri really did explain this in an earlier email
message, but both of us managed to miss it. ;-)
Thanx, Paul
> Regards,
> Srivatsa S. Bhat
>
> > mutex_lock(slab_mutex)
> > _cpu_up()
> > cpu_hotplug_begin()
> > mutex_lock(cpu_hotplug.lock)
> > rcu_barrier()
> > _rcu_barrier()
> > get_online_cpus()
> > mutex_lock(cpu_hotplug.lock)
> > (blocks, CPU 1 has the mutex)
> > __cpu_notify()
> > mutex_lock(slab_mutex)
> >
> > Deadlock.
> >
> > Right?
> >
>
>
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
