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 S933769AbcAMUo6 (ORCPT <rfc822;w@1wt.eu>);
	Wed, 13 Jan 2016 15:44:58 -0500
Received: from casper.infradead.org ([85.118.1.10]:43249 "EHLO
	casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S932082AbcAMUoz (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Wed, 13 Jan 2016 15:44:55 -0500
Date: Wed, 13 Jan 2016 21:44:52 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: mingo@kernel.org, eranian@google.com, linux-kernel@vger.kernel.org,
        vince@deater.net, dvyukov@google.com, andi@firstfloor.org,
        jolsa@redhat.com
Subject: Re: [RFC][PATCH 12/12] perf: Collapse and fix event_function_call()
 users
Message-ID: <20160113204452.GY6373@twins.programming.kicks-ass.net>
References: <20160111162458.427203780@infradead.org>
 <20160111163229.411314288@infradead.org>
 <87r3hl8qxp.fsf@ashishki-desk.ger.corp.intel.com>
 <20160113181016.GU6357@twins.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20160113181016.GU6357@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 Wed, Jan 13, 2016 at 07:10:16PM +0100, Peter Zijlstra wrote:
> On Wed, Jan 13, 2016 at 05:00:50PM +0200, Alexander Shishkin wrote:
> > I think I caught one, below.
> > 
> > Peter Zijlstra <peterz@infradead.org> writes:
> > 
> > > +static int event_function(void *info)
> > > +{
> > > +	struct event_function_struct *efs = info;
> > > +	struct perf_event *event = efs->event;
> > > +	struct perf_event_context *ctx = event->ctx;
> > > +	struct perf_cpu_context *cpuctx = __get_cpu_context(ctx);
> > > +	struct perf_event_context *task_ctx = cpuctx->task_ctx;
> > > +
> > > +	WARN_ON_ONCE(!irqs_disabled());
> > > +
> > > +	/*
> > > +	 * Since we do the IPI call without holding ctx->lock things can have
> > > +	 * changed, double check we hit the task we set out to hit.
> > > +	 *
> > > +	 * If ctx->task == current, we know things must remain valid because
> > > +	 * we have IRQs disabled so we cannot schedule.
> > > +	 */
> > > +	if (ctx->task) {
> > > +		if (ctx->task != current)
> > > +			return -EAGAIN;
> > > +
> > > +		WARN_ON_ONCE(task_ctx != ctx);
> > 
> > Looks like between dropping ctx::lock in event_function_call() and here,
> > cpuctx::task_ctx may still become NULL.
> 
> Hmm yes I think you're right. If the event is being migrated to another
> context concurrently the remove_from_context() might have gone through,
> we'll still be waiting for sync_rcu and then this (say enabled) happens
> and we're looking at a 'dead' context.

Hmm, no. This cannot be, all event_function_call() users should hold
ctx->mutex.