From nobody Sun Dec 28 17:36:00 2025 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 5CD84C10DC1 for ; Thu, 7 Dec 2023 02:38:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1443139AbjLGCiE (ORCPT ); Wed, 6 Dec 2023 21:38:04 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55788 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235202AbjLGCho (ORCPT ); Wed, 6 Dec 2023 21:37:44 -0500 Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 494C0D4B for ; Wed, 6 Dec 2023 18:37:51 -0800 (PST) Received: by smtp.kernel.org (Postfix) with ESMTPSA id E78F6C433C8; Thu, 7 Dec 2023 02:37:50 +0000 (UTC) Received: from rostedt by gandalf with local (Exim 4.97) (envelope-from ) id 1rB4HM-00000001aPg-32s9; Wed, 06 Dec 2023 21:38:20 -0500 Message-ID: <20231207023820.509376496@goodmis.org> User-Agent: quilt/0.67 Date: Wed, 06 Dec 2023 21:37:59 -0500 From: Steven Rostedt To: linux-kernel@vger.kernel.org Cc: Masami Hiramatsu , Mark Rutland , Mathieu Desnoyers , Andrew Morton , stable@vger.kernel.org Subject: [for-linus][PATCH 7/8] ring-buffer: Force absolute timestamp on discard of event References: <20231207023752.712829638@goodmis.org> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" From: "Steven Rostedt (Google)" There's a race where if an event is discarded from the ring buffer and an interrupt were to happen at that time and insert an event, the time stamp is still used from the discarded event as an offset. This can screw up the timings. If the event is going to be discarded, set the "before_stamp" to zero. When a new event comes in, it compares the "before_stamp" with the "write_stamp" and if they are not equal, it will insert an absolute timestamp. This will prevent the timings from getting out of sync due to the discarded event. Link: https://lore.kernel.org/linux-trace-kernel/20231206100244.5130f9b3@ga= ndalf.local.home Cc: stable@vger.kernel.org Cc: Masami Hiramatsu Cc: Mark Rutland Cc: Mathieu Desnoyers Fixes: 6f6be606e763f ("ring-buffer: Force before_stamp and write_stamp to b= e different on discard") Signed-off-by: Steven Rostedt (Google) --- kernel/trace/ring_buffer.c | 19 ++++++++----------- 1 file changed, 8 insertions(+), 11 deletions(-) diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index 43cc47d7faaf..a6da2d765c78 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -3030,22 +3030,19 @@ rb_try_to_discard(struct ring_buffer_per_cpu *cpu_b= uffer, local_read(&bpage->write) & ~RB_WRITE_MASK; unsigned long event_length =3D rb_event_length(event); =20 + /* + * For the before_stamp to be different than the write_stamp + * to make sure that the next event adds an absolute + * value and does not rely on the saved write stamp, which + * is now going to be bogus. + */ + rb_time_set(&cpu_buffer->before_stamp, 0); + /* Something came in, can't discard */ if (!rb_time_cmpxchg(&cpu_buffer->write_stamp, write_stamp, write_stamp - delta)) return false; =20 - /* - * It's possible that the event time delta is zero - * (has the same time stamp as the previous event) - * in which case write_stamp and before_stamp could - * be the same. In such a case, force before_stamp - * to be different than write_stamp. It doesn't - * matter what it is, as long as its different. - */ - if (!delta) - rb_time_set(&cpu_buffer->before_stamp, 0); - /* * If an event were to come in now, it would see that the * write_stamp and the before_stamp are different, and assume --=20 2.42.0