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SubjectRe: [rfc] [patch] mm: Slab - Eliminate lock_cpu_hotplug from slab
On Mon, Oct 30, 2006 at 03:52:06PM +0530, Srivatsa Vaddagiri wrote:
> On Fri, Oct 27, 2006 at 06:19:19PM -0700, Ravikiran G Thirumalai wrote:
> > This patch also takes the cache_chain_sem at kmem_cache_shrink to
> > protect sanity of cpu_online_map at __cache_shrink, as viewed by slab.
> > (kmem_cache_shrink->__cache_shrink->drain_cpu_caches). But, really,
>
> drain_cpu_caches uses on_each_cpu() ..which does a preempt_disable()
> before using the cpu_online_map. That should be a safe enough access to the
> bitmap?

drain_cpu_caches has two call sites -- kmem_cache_destroy and
kmem_cache_shrink.

kmem_cache_shrink:
From what I can gather by looking at the cpu hotplug code, disabling
preemption before iterating over cpu_online_map ensures that a
cpu won't disappear from the bitmask (system). But it does not ensure that a
cpu won't come up right? (I see stop_machine usage in the cpu_down path, but
not in the cpu_up path). But then on closer look I see that on_each_cpu
uses call_lock to protect the cpu_online_map against cpu_online events.
So, yes we don't need to take the cache_chain_sem here.

kmem_cache_destroy:
We still need to stay serialized against cpu online here. I guess
you already know why :)
http://lkml.org/lkml/2004/3/23/80

> > Another note. Looks like a cpu hotplug event can send CPU_UP_CANCELED to
> > a registered subsystem even if the subsystem did not receive CPU_UP_PREPARE.
> > This could be due to a subsystem registered for notification earlier than
> > the current subsystem crapping out with NOTIFY_BAD. Badness can occur with
> > in the CPU_UP_CANCELED code path at slab if this happens (The same would
> > apply for workqueue.c as well). To overcome this, we might have to use either
> > a) a per subsystem flag and avoid handling of CPU_UP_CANCELED, or
> > b) Use a special notifier events like LOCK_ACQUIRE/RELEASE as Gautham was
> > using in his experiments, or
> > c) Do not send CPU_UP_CANCELED to a subsystem which did not receive
> > CPU_UP_PREPARE.
> >
> > I would prefer c).
>
> I think we need both b) and c).
>
> Let me explain.
>
> The need for c) is straightforward.
>
> The need for b) comes from the fact that _cpu_down messes with the
> tsk->cpus_allowed mask (to possibly jump off the dying CPU). This would cause
> sched_getaffinity() to potentially return a false value back to the user and
> hence it was modified to take lock_cpu_hotplug() before reading
> tsk->cpus_allowed.


Maybe I am missing something, but what prevents someone from reading the
wrong tsk->cpus_allowed at (A) below?

static int _cpu_down(unsigned int cpu)
{
...
...
set_cpus_allowed(current, tmp);
----- (A)
mutex_lock(&cpu_bitmask_lock);
p = __stop_machine_run(take_cpu_down, NULL, cpu);
...
}

>
> If we are discarding this whole lock_cpu_hotplug(), then IMO, we should
> use LOCK_ACQUIRE/RELEASE, where ACQUIRE notification is sent *before*
> messing with tsk->cpus_allowed and RELEASE notification sent *after*
> restoring tsk->cpus_allowed (something like below):
>
> @@ -186,13 +186,14 @@ int cpu_down(unsigned int cpu)
> {
> int err = 0;
>
> - mutex_lock(&cpu_add_remove_lock);
> + blocking_notifier_call_chain(&cpu_chain, CPU_LOCK_ACQUIRE,
> + (void *)(long)cpu);
> if (cpu_hotplug_disabled)
> err = -EBUSY;
> else
> err = _cpu_down(cpu);
> -
> - mutex_unlock(&cpu_add_remove_lock);
> + blocking_notifier_call_chain(&cpu_chain, CPU_LOCK_RELEASE,
> + (void *)(long)cpu);
> return err;
> }
>

But, since we send CPU_DOWN_PREPARE at _cpu_down before set_cpus_allowed(),
is it not possible to take the per scheduler subsystem lock at DOWN_PREPARE
and serialize sched_getaffinity with the same per scheduler subsys lock?

Thanks,
Kiran
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