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
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