On Mon, Oct 30, 2006 at 07:51:40PM -0800, Ravikiran G Thirumalai wrote:
> 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.
Yes thats what I thought.
> 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
sure!
> 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)
lock_cpu_hotplug() in sched_getaffinity was supposed to guard from
reading the wrong value you point out. But with recent churn of cpu hotplug
locking, this is broken now.
Ideally, lock_cpu_hotplug() should have taken something equivalent to
cpu_add_remove_lock (breaking cpufreq in the course :), but moving
mutex_lock(&cpu_bitmask_lock) few lines above (before set_cpus_allowed) should
also work in this case.
Alternately, taking a per-subsystem lock in DOWN_PREPARE/LOCK_ACQUIRE
notifications, which is used by sched_getaffinity also, would work
(as you note below).
> 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?
CPU_DOWN_PREPARE is a good enough time to acquire the (scheduler
subsystem) lock. But I am more concerned about when we release that lock.
Releasing at CPU_DEAD/CPU_DOWN_FAILED is too early, since the tasks's
cpus_allowed mask would not have been restored by then. Thats why having a
separate notification to release (and acquire) the lock would make more sense I
thought.
--
Regards,
vatsa
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