On Tue, 2 Oct 2012, Paul E. McKenney wrote:
> Indeed. Slab seems to be doing an rcu_barrier() in a CPU hotplug
> notifier, which doesn't sit so well with rcu_barrier() trying to exclude
> CPU hotplug events. I could go back to the old approach, but it is
> significantly more complex. I cannot say that I am all that happy about
> anyone calling rcu_barrier() from a CPU hotplug notifier because it
> doesn't help CPU hotplug latency, but that is a separate issue.
>
> But the thing is that rcu_barrier()'s assumptions work just fine if either
> (1) it excludes hotplug operations or (2) if it is called from a hotplug
> notifier. You see, either way, the CPU cannot go away while rcu_barrier()
> is executing. So the right way to resolve this seems to be to do the
> get_online_cpus() only if rcu_barrier() is -not- executing in the context
> of a hotplug notifier. Should be fixable without too much hassle...
Sorry, I don't think I understand what you are proposing just yet.
If I understand it correctly, you are proposing to introduce some magic
into _rcu_barrier() such as (pseudocode of course):
if (!being_called_from_hotplug_notifier_callback)
get_online_cpus()
How does that protect from the scenario I've outlined before though?
CPU 0 CPU 1
kmem_cache_destroy()
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)
CPU 0 grabs both locks anyway (it's not running from notifier callback).
CPU 1 grabs both locks as well, as there is no _rcu_barrier() being called
from notifier callback either.
What did I miss?
Thanks,
--
Jiri Kosina
SUSE Labs
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