From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1754420AbbLKPUl (ORCPT <rfc822;w@1wt.eu>);
	Fri, 11 Dec 2015 10:20:41 -0500
Received: from casper.infradead.org ([85.118.1.10]:49262 "EHLO
	casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1752909AbbLKPUk (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Fri, 11 Dec 2015 10:20:40 -0500
Date: Fri, 11 Dec 2015 16:20:37 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        vince@deater.net, eranian@google.com,
        Arnaldo Carvalho de Melo <acme@infradead.org>,
        Mathieu Poirier <mathieu.poirier@linaro.org>
Subject: Re: [PATCH v0 3/5] perf: Introduce instruction trace filtering
Message-ID: <20151211152037.GH6373@twins.programming.kicks-ass.net>
References: <1449840998-29902-1-git-send-email-alexander.shishkin@linux.intel.com>
 <1449840998-29902-4-git-send-email-alexander.shishkin@linux.intel.com>
 <20151211150236.GU6356@twins.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20151211150236.GU6356@twins.programming.kicks-ass.net>
User-Agent: Mutt/1.5.21 (2012-12-30)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Fri, Dec 11, 2015 at 04:02:36PM +0100, Peter Zijlstra wrote:
> On Fri, Dec 11, 2015 at 03:36:36PM +0200, Alexander Shishkin wrote:
> > +static int perf_event_itrace_filters_setup(struct perf_event *event)
> > +{
> > +	int ret;
> > +
> > +	/*
> > +	 * We can't use event_function_call() here, because that would
> > +	 * require ctx::mutex, but one of our callers is called with
> > +	 * mm::mmap_sem down, which would cause an inversion, see bullet
> > +	 * (2) in put_event().
> > +	 */
> > +	do {
> > +		if (READ_ONCE(event->state) != PERF_EVENT_STATE_ACTIVE) {
> > +			ret = event->pmu->itrace_filter_setup(event);
> > +			break;
> 
> So this is tricky, if its not active it can be any moment, there is
> nothing serializing against that.

One solution is to keep a filter generation counter and check that on
pmu::add(), and if mis-matched recompute the hardware setup at that
point.

> > +		}
> > +
> > +		/* matches smp_wmb() in event_sched_in() */
> > +		smp_rmb();
> > +
> > +		ret = cpu_function_call(READ_ONCE(event->oncpu),
> > +					__perf_event_itrace_filters_setup, event);
> 
> This otoh, running with IRQs disabled on the CPU the thing is active on
> guarantees it will not become inactive -- nothing can come in and switch
> it off.
> 
> > +	} while (ret == -EAGAIN);
> > +
> > +	return ret;
> > +}