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Date:   Thu, 22 Nov 2018 13:26:37 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andrew Murray <andrew.murray@arm.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Shawn Guo <shawnguo@kernel.org>,
        Sascha Hauer <s.hauer@pengutronix.de>,
        Will Deacon <will.deacon@arm.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>, x86@kernel.org,
        linux-alpha@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 00/10] perf/core: Generalise event exclusion checking
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On Thu, Nov 22, 2018 at 12:21:43PM +0000, Andrew Murray wrote:
> On Mon, Nov 19, 2018 at 02:08:00PM +0100, Peter Zijlstra wrote:

> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index 84530ab358c3..d76b724177b9 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -9772,6 +9772,14 @@ static int perf_try_init_event(struct pmu *pmu, struct perf_event *event)
> >  	if (ctx)
> >  		perf_event_ctx_unlock(event->group_leader, ctx);
> >  
> > +	if (!ret) {
> > +		if ((pmu->capabilities & PERF_PMU_CAP_EXCLUDE) ||
> > +		    event_has_exclude_flags(event)) {
> > +			event->destroy(event);
> > +			ret = -EINVAL;
> > +		}
> > +	}
> > +
> 
> I don't quite follow this logic. Should that not have been:
> 
> if (!(pmu->capabilities & PERF_PMU_CAP_EXCLUDE) &&
>      event_has_exclude_flags(event)) {
> 
> Meaning that if an event has any exclude flags but the pmu doesn't
> have the capability to handle them then error.

Uhm, yes. Brainfart on my side that.

> If you're happy with my proposed logic, then would it also make
> sense to move this before the call to the pmu->event_init ?

I'm not sure that can work; I think we need ->event_init() first such
that it can -ENOENT. Only after ->event_init() returns success can we be
certain of @pmu.