From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754098AbdH2OBQ (ORCPT ); Tue, 29 Aug 2017 10:01:16 -0400 Received: from mga05.intel.com ([192.55.52.43]:61501 "EHLO mga05.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754081AbdH2OBP (ORCPT ); Tue, 29 Aug 2017 10:01:15 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.41,445,1498546800"; d="scan'208";a="129467633" From: Alexander Shishkin To: Peter Zijlstra Cc: Ingo Molnar , linux-kernel@vger.kernel.org, Alexander Shishkin Subject: [PATCH] perf: Explain perf_sched_mutex Date: Tue, 29 Aug 2017 17:01:03 +0300 Message-Id: <20170829140103.6563-1-alexander.shishkin@linux.intel.com> X-Mailer: git-send-email 2.13.1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org To clarify why atomic_inc_return(&perf_sched_events) is not sufficient and a mutex is needed to order static branch enabling vs the atomic counter increment, this adds a comment with an short explanation. Signed-off-by: Alexander Shishkin --- kernel/events/core.c | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/kernel/events/core.c b/kernel/events/core.c index efe09b8c38..2c8719b635 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -9569,6 +9569,11 @@ static void account_event(struct perf_event *event) inc = true; if (inc) { + /* + * We need the mutex here because static_branch_enable() + * must complete *before* the perf_sched_count increment + * becomes visible. + */ if (atomic_inc_not_zero(&perf_sched_count)) goto enabled; -- 2.14.1