Messages in this thread | | | Date | Tue, 2 Oct 2012 20:44:05 -0700 | From | "Paul E. McKenney" <> | Subject | Re: Lockdep complains about commit 1331e7a1bb ("rcu: Remove _rcu_barrier() dependency on __stop_machine()") |
| |
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? > > > >
| |