From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753418AbdCPUAB (ORCPT ); Thu, 16 Mar 2017 16:00:01 -0400 Received: from mail.kernel.org ([198.145.29.136]:44408 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752784AbdCPT75 (ORCPT ); Thu, 16 Mar 2017 15:59:57 -0400 From: Andy Lutomirski To: X86 ML Cc: "linux-kernel@vger.kernel.org" , Borislav Petkov , Vince Weaver , Andy Lutomirski Subject: [PATCH 2/2] x86/perf: Clarify why x86_pmu_event_mapped() isn't racy Date: Thu, 16 Mar 2017 12:59:40 -0700 Message-Id: <03a1e629063899168dfc4707f3bb6e581e21f5c6.1489694270.git.luto@kernel.org> X-Mailer: git-send-email 2.9.3 In-Reply-To: References: In-Reply-To: References: Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Naively, it looks racy, but mmap_sem saves it. Add a comment and a lockdep assertion. Signed-off-by: Andy Lutomirski --- arch/x86/events/core.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c index 4f564df73b8f..2aa1ad194db2 100644 --- a/arch/x86/events/core.c +++ b/arch/x86/events/core.c @@ -2110,6 +2110,18 @@ static void x86_pmu_event_mapped(struct perf_event *event) if (!(event->hw.flags & PERF_X86_EVENT_RDPMC_ALLOWED)) return; + /* + * This function relies on not being called concurrently in two + * tasks in the same mm. Otherwise one task could observe + * perf_rdpmc_allowed > 1 and return all the way back to + * userspace with CR4.PCE clear while another task is still + * doing on_each_cpu_mask() to propagate CR4.PCE. + * + * For now, this can't happen because all callers hold mmap_sem + * for write. If this changes, we'll need a different solution. + */ + lockdep_assert_held_exclusive(¤t->mm->mmap_sem); + if (atomic_inc_return(¤t->mm->context.perf_rdpmc_allowed) == 1) on_each_cpu_mask(mm_cpumask(current->mm), refresh_pce, NULL, 1); } -- 2.9.3