From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8B9DDC4360F for ; Wed, 20 Feb 2019 18:38:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 648F6222A0 for ; Wed, 20 Feb 2019 18:38:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727486AbfBTSi0 (ORCPT ); Wed, 20 Feb 2019 13:38:26 -0500 Received: from mail.kernel.org ([198.145.29.99]:47282 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727192AbfBTSiK (ORCPT ); Wed, 20 Feb 2019 13:38:10 -0500 Received: from gandalf.local.home (cpe-66-24-58-225.stny.res.rr.com [66.24.58.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 43B8C222A9; Wed, 20 Feb 2019 18:38:10 +0000 (UTC) Received: from rostedt by gandalf.local.home with local (Exim 4.92-RC5) (envelope-from ) id 1gwWkz-0006nx-Dx; Wed, 20 Feb 2019 13:38:09 -0500 Message-Id: <20190220183809.315737571@goodmis.org> User-Agent: quilt/0.65 Date: Wed, 20 Feb 2019 13:38:06 -0500 From: Steven Rostedt To: linux-kernel@vger.kernel.org Cc: Ingo Molnar , Andrew Morton Subject: [for-next][PATCH 29/29] tracing: Comment why cond_snapshot is checked outside of max_lock protection References: <20190220183737.034634614@goodmis.org> MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-15 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: "Steven Rostedt (VMware)" Before setting tr->cond_snapshot, it must be NULL before it can be updated. It can go to NULL when a trace event hist trigger is created or removed, and can only be modified under the max_lock spin lock. But because it can only be set to something other than NULL under both the max_lock spin lock as well as the trace_types_lock, we can perform the check if it is not NULL only under the trace_types_lock and fail out without having to grab the max_lock spin lock. This is very subtle, and deserves a comment. Signed-off-by: Steven Rostedt (VMware) --- kernel/trace/trace.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c index 0460cc0f28fd..2cf3c747a357 100644 --- a/kernel/trace/trace.c +++ b/kernel/trace/trace.c @@ -1116,6 +1116,14 @@ int tracing_snapshot_cond_enable(struct trace_array *tr, void *cond_data, goto fail_unlock; } + /* + * The cond_snapshot can only change to NULL without the + * trace_types_lock. We don't care if we race with it going + * to NULL, but we want to make sure that it's not set to + * something other than NULL when we get here, which we can + * do safely with only holding the trace_types_lock and not + * having to take the max_lock. + */ if (tr->cond_snapshot) { ret = -EBUSY; goto fail_unlock; -- 2.20.1