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SubjectRe: Lockdep complains about commit 1331e7a1bb ("rcu: Remove _rcu_barrier() dependency on __stop_machine()")
On Wed, 3 Oct 2012, Srivatsa S. Bhat wrote:

> >>> 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?

Please look carefully at the scenario again. kmem_cache_destroy() calls
get_online_cpus() before the hotplug notifier even starts. Hence it has no
reason to block there (noone is holding hotplug lock).

*Then* hotplug notifier fires up, succeeds obtaining hotplug lock,
kmem_cache_destroy() calls rcu_barrier in the meantime, and blocks itself
on the hotplug lock there.

Please note that the get_online_cpus() call in kmem_cache_destroy()
doesn't play *any* role in this scenario.

--
Jiri Kosina
SUSE Labs


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