Date: Thu, 18 Jun 2015 09:30:16 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Vince Weaver <vincent.weaver@maine.edu>, linux-kernel@vger.kernel.org,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Stephane Eranian <eranian@gmail.com>
Subject: Re: perf: aux area related crash and warnings
Message-ID: <20150618163016.GB3913@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
References: <alpine.DEB.2.20.1506112305510.2757@vincent-weaver-1.umelst.maine.edu>
 <alpine.DEB.2.20.1506121431190.4444@vincent-weaver-1.umelst.maine.edu>
 <20150615122054.GY3644@twins.programming.kicks-ass.net>
 <87egld2l2o.fsf@ashishki-desk.ger.corp.intel.com>
 <87381rg9zx.fsf@ashishki-desk.ger.corp.intel.com>
 <20150618090954.GB22009@gmail.com>
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On Thu, Jun 18, 2015 at 11:09:55AM +0200, Ingo Molnar wrote:
> 
> * Alexander Shishkin <alexander.shishkin@linux.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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