From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754856AbbEZNeX (ORCPT ); Tue, 26 May 2015 09:34:23 -0400 Received: from mail-oi0-f51.google.com ([209.85.218.51]:33763 "EHLO mail-oi0-f51.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751428AbbEZNeT (ORCPT ); Tue, 26 May 2015 09:34:19 -0400 MIME-Version: 1.0 In-Reply-To: <20150526101237.GK3644@twins.programming.kicks-ass.net> References: <20150522132905.416122812@infradead.org> <20150522133135.353044581@infradead.org> <20150522134056.GG3644@twins.programming.kicks-ass.net> <20150526101237.GK3644@twins.programming.kicks-ass.net> Date: Tue, 26 May 2015 04:46:07 -0700 Message-ID: Subject: Re: [PATCH v2 01/11] perf,x86: Fix event/group validation From: Stephane Eranian To: Peter Zijlstra Cc: Ingo Molnar , Vince Weaver , Jiri Olsa , "Liang, Kan" , LKML , Andrew Hunter , Maria Dimakopoulou Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, May 26, 2015 at 3:12 AM, Peter Zijlstra wrote: > On Tue, May 26, 2015 at 02:24:38AM -0700, Stephane Eranian wrote: >> On Fri, May 22, 2015 at 6:40 AM, Peter Zijlstra wrote: >> > On Fri, May 22, 2015 at 03:29:06PM +0200, Peter Zijlstra wrote: >> >> @@ -788,9 +788,9 @@ int x86_schedule_events(struct cpu_hw_ev >> >> x86_pmu.start_scheduling(cpuc); >> >> >> >> for (i = 0, wmin = X86_PMC_IDX_MAX, wmax = 0; i < n; i++) { >> >> + cpuc->event_constraint[i] = NULL; >> > >> > ^^^ that is new, which is esp. important in light of the >> > intel_get_event_constraints() hunk below, which would happily continue >> > life with a garbage constraint. >> > >> You've moved the constraint list from event to cpuc. Yet, it is still >> an array of pointers >> to constraints. So here you are saying, that in the case validate_group() is >> preempted and there is a context switch, there is still a risk of >> overwriting the >> constraint? I don't see how because validate_group() is using a fake_cpuc. >> So yes, the cpuc->event_constraint[] array is modified but it is not the same >> as the actual cpuc used by non-validate code. Or am I still missing something? >> >> When using dynamic constraints, we already have constraint storage in cpuc >> (to avoid calling kmalloc() in ctxsw context). Thus, I am wondering if it would >> not be easier to always use cpuc for constraint storage (no more pointers). > > No; the problem here is repeated use of the cpuc (the real one). Say one > scheduling run installs a constraint pointer for event i. Then event i > gets removed and another installed in the same spot. > > Then the next scheduling run will pick up the old pointer in > intel_get_event_constraints() as a base for the new one. > But where is the code that says: skip reinstalling the constraint in intel_get_event_constraints() because there is already a (stale) one? I don't see where that is.