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 4B1DF323D; Tue, 27 May 2025 03:54:46 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1748318087; cv=none; b=eyh9JV6W+oV30TPvnOkU5p+7jVoePPWgFllER/0AmHkKefQRuseeUpOWyClhffE6BQ13LD/lSZt4Yl4u85/xT/1q9Zz1Zbt1VLXh6YOX6QejtwzmfGzkS3K3s/3svaNoG/EV7qX9duw/DCwKrZbkYWPJjToA4QSsTe0OZ8ruCyQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1748318087; c=relaxed/simple; bh=Ng61/aB84nxCgAq7cyaS69ZYzC+Riw+xgfzDZXLPnlo=; h=Date:From:To:Cc:Subject:Message-Id:In-Reply-To:References: Mime-Version:Content-Type; b=pniusO7iE5+OGKlyL/0GbF+sji6mYhSGaWHYoQQa4S32MdRf+tavmnqa9moruIyHyA5lwb1Fw15HUvIzzHAAdqkXTYDxjCD/aUEguQbYOUxXko+dLpAgutfJBOq6ePW9dSFU588iKP94cInmErEehFjdpNtlD3c8pemRzenxHsU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=eKwc+h+S; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="eKwc+h+S" Received: by smtp.kernel.org (Postfix) with ESMTPSA id CF545C4CEEA; Tue, 27 May 2025 03:54:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1748318086; bh=Ng61/aB84nxCgAq7cyaS69ZYzC+Riw+xgfzDZXLPnlo=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=eKwc+h+SFGHxjkq7bmD9++4GGcSQ075Ow5aRDjeYMPN7o+G7Q/OX1d29Wu0+ZtlpT 5OZeTYvMdO3oFHEDd4o6dyQSSaSyLzcRfjsBO8LsWannbxvLU32/som5SMI7ejIBKk IgmsOKs1woRkcH89ueNHMsSoNCYb5lP9pqSjveJY1ZXiCbGmQ1+YwRZAguQTbgm6cL CwJMft8wYK2npIG41Y0uYf/5JdQPAi9ff+0s29Ettsd3zVlG9fz17ikF6O/U8XYKwP p2TplWx7Z4oWJisH8ZA1xSIfbOCSwVJB45AajXA5uOWIdgcmXG0LMyPAgHekZ+PqOL cRXAscjm6aV1Q== Date: Tue, 27 May 2025 12:54:44 +0900 From: Masami Hiramatsu (Google) To: Masami Hiramatsu (Google) Cc: Steven Rostedt , Mathieu Desnoyers , linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org Subject: Re: [PATCH v3 1/2] tracing: ring_buffer: Rewind persistent ring buffer when reboot Message-Id: <20250527125444.80e59e659a7f087fed3c6fc9@kernel.org> In-Reply-To: <20250527121549.2d40608e9463199d85bf9ba4@kernel.org> References: <174792927500.496143.10668210464102033012.stgit@mhiramat.tok.corp.google.com> <174792928413.496143.17979267710069360561.stgit@mhiramat.tok.corp.google.com> <20250523165425.0275c9a1@gandalf.local.home> <20250523172857.02ab4a75@gandalf.local.home> <20250526120342.cdcfca9ac3c2161d48c03af5@kernel.org> <20250527091743.d036018195b7668aea0753c0@kernel.org> <20250527121549.2d40608e9463199d85bf9ba4@kernel.org> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Tue, 27 May 2025 12:15:49 +0900 Masami Hiramatsu (Google) wrote: > > What about this? It seems if we keep the reader->page->commit, when we > > catch up the tail page, rb_get_reader_page() returns new reader (== tail) > > because reader->read(==0) < reader->page->commit. But we should not return > > new reader. > > > > > > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c > > index 5034bae02f08..179142db9586 100644 > > --- a/kernel/trace/ring_buffer.c > > +++ b/kernel/trace/ring_buffer.c > > @@ -5522,6 +5522,12 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer) > > cpu_buffer->last_overrun = overwrite; > > } > > > > + /* But if we catch up to the tail, do not return reader page. */ > > + if (cpu_buffer->commit_page == cpu_buffer->reader_page) { > > > Hmm, I found this might not work if it is called twice. > Maybe rb_get_reader_page() can check the reader_page->ts > head_page->ts > and return NULL (or reader_page if read < commit) at first. Here is what I meant. As far as I ran my test, it looks good (it prevents over-read by `cat per_cpu/cpu0/trace_pipe_raw`) diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index 5034bae02f08..de1831eb3446 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -5405,6 +5405,7 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer) unsigned long flags; int nr_loops = 0; bool ret; + u64 ts; local_irq_save(flags); arch_spin_lock(&cpu_buffer->lock); @@ -5423,6 +5424,18 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer) reader = cpu_buffer->reader_page; + /* + * Now the page->commit is not cleared when it read. + * Check whether timestamp is newer instead. We also don't + * care the head_page is overwritten. In that case, timestamp + * should be newer than reader timestamp too. + */ + ts = cpu_buffer->head_page->page->time_stamp; + if (ts < reader->page->time_stamp) { + reader = NULL; + goto out; + } + /* If there's more to read, return this page */ if (cpu_buffer->reader_page->read < rb_page_size(reader)) goto out; -- Masami Hiramatsu (Google)