linux-kernel - Re: perf: aux area related crash and warnings

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Date:	Thu, 18 Jun 2015 09:30:16 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Vince Weaver <vincent.weaver@...ne.edu>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Stephane Eranian <eranian@...il.com>
Subject: Re: perf: aux area related crash and warnings

On Thu, Jun 18, 2015 at 11:09:55AM +0200, Ingo Molnar wrote:
> 
> * Alexander Shishkin <alexander.shishkin@...ux.intel.com> wrote:
> 
> > +void rb_free_aux(struct ring_buffer *rb)
> > +{
> > +	/*
> > +	 * hold rb::refcount to make sure rb doesn't disappear
> > +	 * before aux pages are freed
> > +	 */
> > +	if (WARN_ON_ONCE(!atomic_inc_not_zero(&rb->refcount)))
> > +		return;
> > +
> > +	if (atomic_dec_and_test(&rb->aux_refcount))
> > +		call_rcu(&rb->rcu_head, rb_free_rcu);
> > +	else
> > +		ring_buffer_put(rb);	/* matches the increment above */
> 
> Is call_rcu() NMI-safe? I don't think so ...

Definitely not!  ;-)

> I think the life time rules of this object are really messed up if they can be 
> freed from any fast path. How come the freeing can happen in NMI context? 
> Shouldn't the hardware first stop, then we can free things from the system call 
> path, or so?
> 
> Thanks,
> 
> 	Ingo
> 

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