On 10/03/2012 09:14 AM, Paul E. McKenney wrote: > 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.
Agreed. > Then having kmem_cache_destroy() call > rcu_barrier() under the lock Ah, that's where I disagree. kmem_cache_destroy() *cannot* proceed at this point in time, because it has invoked get_online_cpus()! It simply cannot be running past that point in the presence of a running hotplug notifier! So, kmem_cache_destroy() should have been sleeping on the hotplug lock, waiting for the notifier to release it, no? > 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. ;-) > Maybe I'm just being blind, sorry! ;-) Regards, Srivatsa S. Bhat > 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/
