Re: [PATCH v0 3/5] perf: Introduce instruction trace filtering

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Dec 11, 2015 at 05:01:36PM +0100, Peter Zijlstra wrote:
> On Fri, Dec 11, 2015 at 05:48:03PM +0200, Alexander Shishkin wrote:
> 
> > > We can always call the validation thing, we must not call the program
> > > thing !ACTIVE is a clear and simple rule.
> > 
> > Ah, but pmu::itrace_filter_setup() does not touch the hardware,
> > pmu::start() does. The former keeps an array of, say, MSR values ready
> > for programming in event::hw and the latter actually writes the MSRs. So
> > the above example should not be a problem.
> > 
> > So in a way validation and programming are split already. And PT, for
> > example, won't have it any other way, you can only program stuff into
> > the registers while tracing is disabled.
> 
> Yes, I just read that in the last patch. If however we fold the whole
> stop/start bits into it, that fails again.
> 
> Hmm.. lemme ponder a bit.

Nope, it doesn't matter. Either way around you need serialization.

Because while, as proposed, pmu::itrace_filter_setup() does not modify
the hardware state, it does modify event state. So it needs to be
serialized against concurrent pmu::add().