On Wed, Oct 03, 2012 at 01:48:21AM +0200, Jiri Kosina wrote:
> 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?
You didn't miss anything, I was suffering a failure to read carefully.
So my next stupid question is "Why can't kmem_cache_destroy drop
slab_mutex early?" like the following:
void kmem_cache_destroy(struct kmem_cache *cachep)
{
BUG_ON(!cachep || in_interrupt());
/* Find the cache in the chain of caches. */
get_online_cpus();
mutex_lock(&slab_mutex);
/*
* the chain is never empty, cache_cache is never destroyed
*/
list_del(&cachep->list);
if (__cache_shrink(cachep)) {
slab_error(cachep, "Can't free all objects");
list_add(&cachep->list, &slab_caches);
mutex_unlock(&slab_mutex);
put_online_cpus();
return;
}
mutex_unlock(&slab_mutex);
if (unlikely(cachep->flags & SLAB_DESTROY_BY_RCU))
rcu_barrier();
__kmem_cache_destroy(cachep);
put_online_cpus();
}
Or did I miss some reason why __kmem_cache_destroy() needs that lock?
Looks to me like it is just freeing now-disconnected memory.
Thanx, Paul
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