Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752494AbdCNP7H (ORCPT ); Tue, 14 Mar 2017 11:59:07 -0400 Received: from mail-vk0-f50.google.com ([209.85.213.50]:33072 "EHLO mail-vk0-f50.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750988AbdCNP66 (ORCPT ); Tue, 14 Mar 2017 11:58:58 -0400 MIME-Version: 1.0 In-Reply-To: References: <20170313165507.GJ3312@twins.programming.kicks-ass.net> From: Andy Lutomirski Date: Tue, 14 Mar 2017 08:58:36 -0700 Message-ID: Subject: Re: perf: race with automatic rdpmc() disabling To: Andy Lutomirski Cc: Peter Zijlstra , Vince Weaver , "linux-kernel@vger.kernel.org" , Ingo Molnar , Arnaldo Carvalho de Melo Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1291 Lines: 37 On Mon, Mar 13, 2017 at 2:05 PM, Andy Lutomirski wrote: > On Mon, Mar 13, 2017 at 9:55 AM, Peter Zijlstra wrote: >> 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(). >> >> > > One thing I don't get: isn't mmap_sem held for write the whole time? mmap_sem is indeed held, so my theory is wrong. I can reproduce it, but I don't see the bug yet... --Andy -- Andy Lutomirski AMA Capital Management, LLC