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=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_2 autolearn=no 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 E459FC433E2 for ; Tue, 15 Sep 2020 18:34:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B446920732 for ; Tue, 15 Sep 2020 18:34:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727926AbgIOSe2 (ORCPT ); Tue, 15 Sep 2020 14:34:28 -0400 Received: from mail.kernel.org ([198.145.29.99]:40912 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727970AbgIOSNI (ORCPT ); Tue, 15 Sep 2020 14:13:08 -0400 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 D631F20936; Tue, 15 Sep 2020 18:13:05 +0000 (UTC) Date: Tue, 15 Sep 2020 14:13:04 -0400 From: Steven Rostedt To: Gaurav Kohli Cc: mingo@redhat.com, linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org Subject: Re: [PATCH] trace: Fix race in trace_open and buffer resize call Message-ID: <20200915141304.41fa7c30@gandalf.local.home> In-Reply-To: <08d6f338-3be3-c5a2-ba4b-0116de9672c2@codeaurora.org> References: <1599199797-25978-1-git-send-email-gkohli@codeaurora.org> <2fe2a843-e2b5-acf8-22e4-7231d24a9382@codeaurora.org> <20200915092353.5b805468@gandalf.local.home> <08d6f338-3be3-c5a2-ba4b-0116de9672c2@codeaurora.org> X-Mailer: Claws Mail 3.17.3 (GTK+ 2.24.32; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 15 Sep 2020 22:53:32 +0530 Gaurav Kohli wrote: > On 9/15/2020 6:53 PM, Steven Rostedt wrote: > > On Tue, 15 Sep 2020 10:38:03 +0530 > > Gaurav Kohli wrote: > > > >> > >> >>> +void ring_buffer_mutex_release(struct trace_buffer *buffer) > >> >>> +{ > >> >>> + mutex_unlock(&buffer->mutex); > >> >>> +} > >> >>> +EXPORT_SYMBOL_GPL(ring_buffer_mutex_release); > >> > > >> > I really do not like to export these. > >> > > >> > >> Actually available reader lock is not helping > >> here(&cpu_buffer->reader_lock), So i took ring buffer mutex lock to > >> resolve this(this came on 4.19/5.4), in latest tip it is trace buffer > >> lock. Due to this i have exported api. > > > > I'm saying, why don't you take the buffer->mutex in the > > ring_buffer_reset_online_cpus() function? And remove all the protection in > > tracing_reset_online_cpus()? > > Yes, got your point. then we can avoid export. Actually we are seeing > issue in older kernel like 4.19/4.14/5.4 and there below patch was not > present in stable branches: > > ommit b23d7a5f4a07 ("ring-buffer: speed up buffer resets by > > avoiding synchronize_rcu for each CPU") If you mark this patch for stable, you can add: Depends-on: b23d7a5f4a07 ("ring-buffer: speed up buffer resets by avoiding synchronize_rcu for each CPU") > > Actually i have also thought to take mutex lock in ring_buffer_reset_cpu > while doing individual cpu reset, but this could cause another problem: Hmm, I think we should also take the buffer lock in the reset_cpu() call too, and modify tracing_reset_cpu() the same way. > > Different cpu buffer may have different state, so i have taken lock in > tracing_reset_online_cpus. Why would different states be an issue in synchronizing? -- Steve > > > > void tracing_reset_online_cpus(struct array_buffer *buf) > > { > > struct trace_buffer *buffer = buf->buffer; > > > > if (!buffer) > > return; > > > > buf->time_start = buffer_ftrace_now(buf, buf->cpu); > > > > ring_buffer_reset_online_cpus(buffer); > > } > > > > The reset_online_cpus() is already doing the synchronization, we don't need > > to do it twice. > > > > I believe commit b23d7a5f4a07 ("ring-buffer: speed up buffer resets by > > avoiding synchronize_rcu for each CPU") made the synchronization in > > tracing_reset_online_cpus() obsolete. > > > > -- Steve > > > > Yes, with above patch no need to take lock in tracing_reset_online_cpus.