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Date: Fri, 28 Feb 2014 13:17:33 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>,
        Vince Weaver <vincent.weaver@maine.edu>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Linux Kernel <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>
Subject: Re: perf_fuzzer compiled for x32 causes reboot
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On Fri, Feb 28, 2014 at 09:54:09PM +0100, Peter Zijlstra wrote:
> On Fri, Feb 28, 2014 at 03:47:16PM -0500, Steven Rostedt wrote:
> > > > I'll try your patch momentarily, first I had some other changes I started 
> > > > running before I left work (for some reason it recompiled the whole 
> > > > kernel).
> > > > 
> > > > 8: function:             perf_output_begin
> > > > 8: bprint:               perf_output_begin: VMW: event type 2 config 2a st: 2c3e
> > > > 8: bputs:                perf_output_begin: VMW: before rcu_dereference
> > > > 9: function:             __do_page_fault
> > > > 9: function:                down_read_trylock
> > > > 9: function:                _cond_resched
> > > > 9: function:                find_vma
> > > > 
> > > > so it looks like the fault happens 
> > > > 
> > > > rcu_read_lock();
> > > > 
> > > > 116         /*
> > > > 117          * For inherited events we send all the output towards the parent.
> > > > 118          */
> > > > 119         if (event->parent)
> > > > 120                 event = event->parent;
> > > > 121 
> > > > 
> > > > somewhere between here
> > > > 
> > > > 122         rb = rcu_dereference(event->rb);
> > > > 123         if (unlikely(!rb))
> > > > 124                 goto out;
> > > > 
> > > > and here
> > > > 
> > > > 125 
> > > > 126         if (unlikely(!rb->nr_pages))
> > > > 127                 goto out;
> > > > 
> > > > although if rcu locks do anything to turn off tracing then this could be 
> > > > suspect.
> > > 
> > > The most likely suspect is of course event->rb in the rcu_dereference.
> > > I have to defer to Steven on how rcu_read_lock() and rcu_read_unlock()
> > > currently interact with tracing.  ;-)
> > 
> > These are all perf related. You'll need to defer to Peter Zijlstra ;-)
> 
> I'm lost.. :/
> 
> pretty much all perf objects are RCU freed. 

This code isn't running in idle context is it?  If so, RCU will happily
free out from under it.  CONFIG_PROVE_RCU should detect this sort of thing,
though.

							Thanx, Paul