linux-kernel - Re: Lockdep complains about commit 1331e7a1bb ("rcu: Remove _rcu_barrier() dependency on __stop

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Message-ID: <alpine.LNX.2.00.1210031013110.23544@pobox.suse.cz>
Date:	Wed, 3 Oct 2012 10:19:11 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	paulmck@...ux.vnet.ibm.com,
	"Paul E. McKenney" <paul.mckenney@...aro.org>,
	Josh Triplett <josh@...htriplett.org>,
	linux-kernel@...r.kernel.org
Subject: Re: Lockdep complains about commit 1331e7a1bb ("rcu: Remove
 _rcu_barrier() dependency on __stop_machine()")

On Wed, 3 Oct 2012, Srivatsa S. Bhat wrote:

> On 10/03/2012 01:13 PM, Jiri Kosina wrote:
> > 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).
> > 
> 
> Agreed.
> 
> > *Then* hotplug notifier fires up, succeeds obtaining hotplug lock, 
> 
> Ah, that's the problem! The hotplug reader-writer synchronization is not just
> via a simple mutex. Its a refcount underneath. If kmem_cache_destroy() incremented
> the refcount, the hotplug-writer (cpu_up) will release the hotplug lock immediately
> and try again. IOW, a hotplug-reader (kmem_cache_destroy()) and a hotplug-writer
> (cpu_up) can *NEVER* run concurrently. If they do, we are totally screwed!
> 
> 
> Take a look at the hotplug lock acquire function at the writer side:
> 
> static void cpu_hotplug_begin(void)
> {
>         cpu_hotplug.active_writer = current;
> 
>         for (;;) {
>                 mutex_lock(&cpu_hotplug.lock);
>                 if (likely(!cpu_hotplug.refcount))   <================ This one!
>                         break;
>                 __set_current_state(TASK_UNINTERRUPTIBLE);
>                 mutex_unlock(&cpu_hotplug.lock);
>                 schedule();
>         }   
> }

I acutally just came to the same conclusion (7 hours of sleep later, the 
mind indeed seems to be brighter ... what a poet I am).

Lockdep doesn't know about this semantics of cpu_hotplug_begin(), and 
therefore gets confused by the fact that mutual exclusion is actually 
achieved through the refcount instead of mutex (and the same apparently 
happened to me).

So right, now I agree that the deadlock scenario I have come up with is 
indeed bogus (*), and we just have to annotate this fact to lockdep 
somehow.

And I actually believe that moving the slab_mutex around in 
kmem_cache_destroy() is a good anotation (maybe worth a separate comment 
in the code), as it not only makes the lockdep false positive go away, but 
it also reduces the mutex hold time.

(*) I have seen machine locking hard reproducibly, but that was only with 
additional Paul's patch, so I guess the lock order there actually was 
wrong

Thanks!

-- 
Jiri Kosina
SUSE Labs
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