Re: [PATCH 00/10] perf/core: Generalise event exclusion checking

From: Peter Zijlstra <peterz@infradead.org>
To: Andrew Murray <andrew.murray@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	Sascha Hauer <s.hauer@pengutronix.de>,
	x86@kernel.org, Will Deacon <will.deacon@arm.com>,
	linuxppc-dev@lists.ozlabs.org,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
	Borislav Petkov <bp@alien8.de>,
	linux-alpha@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
	Shawn Guo <shawnguo@kernel.org>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 00/10] perf/core: Generalise event exclusion checking
Date: Thu, 22 Nov 2018 13:26:37 +0100	[thread overview]
Message-ID: <20181122122637.GS2131@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20181122122142.GI42987@e119886-lin.cambridge.arm.com>

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.