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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 80867C6FD20 for ; Fri, 24 Mar 2023 19:24:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231608AbjCXTYK (ORCPT ); Fri, 24 Mar 2023 15:24:10 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:45758 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231596AbjCXTYH (ORCPT ); Fri, 24 Mar 2023 15:24:07 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [IPv6:2604:1380:4601:e00::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B319711E95; Fri, 24 Mar 2023 12:23:16 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 9DF67B82546; Fri, 24 Mar 2023 19:23:14 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id B677BC433D2; Fri, 24 Mar 2023 19:23:12 +0000 (UTC) Date: Fri, 24 Mar 2023 15:23:09 -0400 From: Steven Rostedt To: Zheng Yejian Cc: , , Subject: Re: [PATCH] ring-buffer: Fix race while reader and writer are on the same page Message-ID: <20230324152309.134f361a@gandalf.local.home> In-Reply-To: <20230324125037.2719020-1-zhengyejian1@huawei.com> References: <20230324125037.2719020-1-zhengyejian1@huawei.com> X-Mailer: Claws Mail 3.17.8 (GTK+ 2.24.33; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-trace-kernel@vger.kernel.org On Fri, 24 Mar 2023 20:50:37 +0800 Zheng Yejian wrote: > Fixes: 77ae365eca89 ("ring-buffer: make lockless") > Signed-off-by: Zheng Yejian > --- > kernel/trace/ring_buffer.c | 14 +++++++++++++- > 1 file changed, 13 insertions(+), 1 deletion(-) > > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c > index c6f47b6cfd5f..79fd5e10ee05 100644 > --- a/kernel/trace/ring_buffer.c > +++ b/kernel/trace/ring_buffer.c > @@ -2942,6 +2942,13 @@ rb_update_event(struct ring_buffer_per_cpu *cpu_buffer, > event->array[0] = length; > } else > event->type_len = DIV_ROUND_UP(length, RB_ALIGNMENT); > + > + /* > + * The 'event' may be reserved from the page which is reading > + * by reader, make sure 'event' is completely updated before > + * reader_page->page->commit being set. > + */ > + smp_wmb(); This isn't the place to put this. We only care before the commit is updated, not at *ever* update to the event (this can be called several times before a commit). If we need to add a smp_wmb() it's best to be in rb_set_commit_to_write() > } > > static unsigned rb_calculate_event_length(unsigned length) > @@ -4684,7 +4691,12 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer) > > /* > * Make sure we see any padding after the write update > - * (see rb_reset_tail()) > + * (see rb_reset_tail()). > + * > + * In addition, writer may be writing on the reader page > + * if the page has not been fully filled, so the read barrier > + * is also needed to make sure we see the completely updated > + * event that reserved by writer (see rb_update_event()). > */ > smp_rmb(); > I think we want this instead: diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index 2d5c3caff32d..22d05cd04a3a 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -3092,6 +3092,10 @@ rb_set_commit_to_write(struct ring_buffer_per_cpu *cpu_buffer) if (RB_WARN_ON(cpu_buffer, rb_is_reader_page(cpu_buffer->tail_page))) return; + /* + * No need for a memory barrier here, as the update + * of the tail_page did it for this page. + */ local_set(&cpu_buffer->commit_page->page->commit, rb_page_write(cpu_buffer->commit_page)); rb_inc_page(&cpu_buffer->commit_page); @@ -3101,6 +3105,8 @@ rb_set_commit_to_write(struct ring_buffer_per_cpu *cpu_buffer) while (rb_commit_index(cpu_buffer) != rb_page_write(cpu_buffer->commit_page)) { + /* Make sure the readers see the content of what is committed. */ + smp_wmb(); local_set(&cpu_buffer->commit_page->page->commit, rb_page_write(cpu_buffer->commit_page)); RB_WARN_ON(cpu_buffer, @@ -4676,7 +4682,12 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer) /* * Make sure we see any padding after the write update - * (see rb_reset_tail()) + * (see rb_reset_tail()). + * + * In addition, a writer may be writing on the reader page + * if the page has not been fully filled, so the read barrier + * is also needed to make sure we see the completely updated + * event reserved by the writer (see rb_tail_page_update()). */ smp_rmb();