From mboxrd@z Thu Jan 1 00:00:00 1970 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 B68951D13A; Mon, 18 Dec 2023 13:59:21 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="HZl0ImaV" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3CCC0C433C7; Mon, 18 Dec 2023 13:59:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1702907961; bh=bMcqL8Ojjr4fbUvEFNn5c1lzgEBjqyXKg41KlUAnQu4=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=HZl0ImaVFgOIViI0gFFyCDIiVrroOALJHxhUXgwbHydUt2eqVwLAJIDrDU7drn8vb XJRArsGrcNXFJo56JEsgPyaQP/+mdqthxu5Rzp7cv3cOYEww0ZJ26Np1Bek++bjm5n CsbDoz/f5c4qAxFy0feGAzMkVqu/EZn9TD12U3OU= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Mark Rutland , Mathieu Desnoyers , "Masami Hiramatsu (Google)" , "Steven Rostedt (Google)" Subject: [PATCH 6.1 097/106] ring-buffer: Do not update before stamp when switching sub-buffers Date: Mon, 18 Dec 2023 14:51:51 +0100 Message-ID: <20231218135059.199953469@linuxfoundation.org> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20231218135055.005497074@linuxfoundation.org> References: <20231218135055.005497074@linuxfoundation.org> User-Agent: quilt/0.67 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.1-stable review patch. If anyone has any objections, please let me know. ------------------ From: Steven Rostedt (Google) commit 9e45e39dc249c970d99d2681f6bcb55736fd725c upstream. 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 happens, 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@gandalf.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) Signed-off-by: Greg Kroah-Hartman --- kernel/trace/ring_buffer.c | 9 +-------- 1 file changed, 1 insertion(+), 8 deletions(-) --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -3601,14 +3601,7 @@ __rb_reserve_next(struct ring_buffer_per /* 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 = rb_time_read(&cpu_buffer->before_stamp, &info->before); - a_ok = rb_time_read(&cpu_buffer->write_stamp, &info->after); - if (a_ok && b_ok && info->before != 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); }