Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752864AbdCMQzX (ORCPT ); Mon, 13 Mar 2017 12:55:23 -0400 Received: from bombadil.infradead.org ([65.50.211.133]:48384 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750865AbdCMQzN (ORCPT ); Mon, 13 Mar 2017 12:55:13 -0400 Date: Mon, 13 Mar 2017 17:55:07 +0100 From: Peter Zijlstra To: Andy Lutomirski Cc: Vince Weaver , "linux-kernel@vger.kernel.org" , Ingo Molnar , Arnaldo Carvalho de Melo Subject: Re: perf: race with automatic rdpmc() disabling Message-ID: <20170313165507.GJ3312@twins.programming.kicks-ass.net> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 860 Lines: 23 On Mon, Mar 13, 2017 at 09:44:02AM -0700, Andy Lutomirski wrote: > static void x86_pmu_event_mapped(struct perf_event *event) > { > if (!(event->hw.flags & PERF_X86_EVENT_RDPMC_ALLOWED)) > return; > > if (atomic_inc_return(¤t->mm->context.perf_rdpmc_allowed) == 1) > > <-- thread 1 stalls here > > on_each_cpu_mask(mm_cpumask(current->mm), refresh_pce, NULL, 1); > } > > Suppose you start with perf_rdpmc_allowed == 0. Thread 1 runs > x86_pmu_event_mapped and gets preempted (or just runs slowly) where I > marked. Then thread 2 runs the whole function, does *not* update CR4, > returns to userspace, and GPFs. > > The big hammer solution is to stick a per-mm mutex around it. Let me > ponder whether a smaller hammer is available. Reminds me a bit of what we ended up with in kernel/jump_label.c:static_key_slow_inc().