From nobody Sat Dec 27 20:37:51 2025 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 1DEAC14A8A; Sat, 16 Dec 2023 04:21:52 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id A620AC4339A; Sat, 16 Dec 2023 04:21:52 +0000 (UTC) Received: from rostedt by gandalf with local (Exim 4.97) (envelope-from ) id 1rEMCJ-00000002yDG-2W0E; Fri, 15 Dec 2023 23:22:43 -0500 Message-ID: <20231216042243.383892237@goodmis.org> User-Agent: quilt/0.67 Date: Fri, 15 Dec 2023 23:22:20 -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 06/15] ring-buffer: Do not update before stamp when switching sub-buffers References: <20231216042214.905262999@goodmis.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" From: "Steven Rostedt (Google)" The ring buffer timestamps are synchronized by two timestamp placeholders. One is the "before_stamp" and the other is the "write_stamp" (sometimes referred to as the "after stamp" but only in the comments. These two stamps are key to knowing how to handle nested events coming in with a lockless system. When moving across sub-buffers, the before stamp is updated but the write stamp is not. There's an effort to put back the before stamp to something that seems logical in case there's nested events. But as the current event is about to cross sub-buffers, and so will any new nested event that happen= s, updating the before stamp is useless, and could even introduce new race conditions. The first event on a sub-buffer simply uses the sub-buffer's timestamp and keeps a "delta" of zero. The "before_stamp" and "write_stamp" are not used in the algorithm in this case. There's no reason to try to fix the before_stamp when this happens. As a bonus, it removes a cmpxchg() when crossing sub-buffers! Link: https://lore.kernel.org/linux-trace-kernel/20231211114420.36dde01b@ga= ndalf.local.home Cc: stable@vger.kernel.org Cc: Mark Rutland Cc: Mathieu Desnoyers Fixes: a389d86f7fd09 ("ring-buffer: Have nested events still record running= time stamp") Reviewed-by: Masami Hiramatsu (Google) Signed-off-by: Steven Rostedt (Google) --- kernel/trace/ring_buffer.c | 9 +-------- 1 file changed, 1 insertion(+), 8 deletions(-) diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index dcd47895b424..c7abcc215fe2 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -3607,14 +3607,7 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_bu= ffer, =20 /* See if we shot pass the end of this buffer page */ if (unlikely(write > BUF_PAGE_SIZE)) { - /* before and after may now different, fix it up*/ - b_ok =3D rb_time_read(&cpu_buffer->before_stamp, &info->before); - a_ok =3D rb_time_read(&cpu_buffer->write_stamp, &info->after); - if (a_ok && b_ok && info->before !=3D info->after) - (void)rb_time_cmpxchg(&cpu_buffer->before_stamp, - info->before, info->after); - if (a_ok && b_ok) - check_buffer(cpu_buffer, info, CHECK_FULL_PAGE); + check_buffer(cpu_buffer, info, CHECK_FULL_PAGE); return rb_move_tail(cpu_buffer, tail, info); } =20 --=20 2.42.0