From nobody Mon Apr 6 18:06:53 2026 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 E9C8DC433F5 for ; Wed, 5 Oct 2022 15:38:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230399AbiJEPh6 (ORCPT ); Wed, 5 Oct 2022 11:37:58 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55640 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229819AbiJEPhz (ORCPT ); Wed, 5 Oct 2022 11:37:55 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E17A0786E0 for ; Wed, 5 Oct 2022 08:37:54 -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 dfw.source.kernel.org (Postfix) with ESMTPS id 70D6061728 for ; Wed, 5 Oct 2022 15:37:54 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 58D43C433D6; Wed, 5 Oct 2022 15:37:53 +0000 (UTC) Date: Wed, 5 Oct 2022 11:37:57 -0400 From: Steven Rostedt To: LKML Cc: Masami Hiramatsu , Ross Zwisler , Andrew Morton Subject: [PATCH v2] tracing: Do not free snapshot if tracer is on cmdline Message-ID: <20221005113757.041df7fe@gandalf.local.home> X-Mailer: Claws Mail 3.17.8 (GTK+ 2.24.33; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" From: "Steven Rostedt (Google)" The ftrace_boot_snapshot and alloc_snapshot cmdline options allocate the snapshot buffer at boot up for use later. The ftrace_boot_snapshot in particular requires the snapshot to be allocated because it will take a snapshot at the end of boot up allowing to see the traces that happened during boot so that it's not lost when user space takes over. When a tracer is registered (started) there's a path that checks if it requires the snapshot buffer or not, and if it does not and it was allocated it will do a synchronization and free the snapshot buffer. This is only required if the previous tracer was using it for "max latency" snapshots, as it needs to make sure all max snapshots are complete before freeing. But this is only needed if the previous tracer was using the snapshot buffer for latency (like irqoff tracer and friends). But it does not make sense to free it, if the previous tracer was not using it, and the snapshot was allocated by the cmdline parameters. This basically takes away the point of allocating it in the first place! Note, the allocated snapshot worked fine for just trace events, but fails when a tracer is enabled on the cmdline. Further investigation, this goes back even further and it does not require a tracer on the cmdline to fail. Simply enable snapshots and then enable a tracer, and it will remove the snapshot. Cc: Masami Hiramatsu Cc: Andrew Morton Cc: stable@vger.kernel.org Fixes: 45ad21ca5530 ("tracing: Have trace_array keep track if snapshot buff= er is allocated") Reported-by: Ross Zwisler Signed-off-by: Steven Rostedt (Google) Acked-by: Masami Hiramatsu (Google) Tested-by: Ross Zwisler --- Changes since v1: https://lkml.kernel.org/r/20221004180452.6cf967f8@gandalf= .local.home - Didn't notice that tr->current_trace was set to &nop_trace just before testing if the last tracer had use_max_tr. Which would always be false as it was testing the nop_trace.use_max_tr which is false. kernel/trace/trace.c | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-) diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c index def721de68a0..47a44b055a1d 100644 --- a/kernel/trace/trace.c +++ b/kernel/trace/trace.c @@ -6428,12 +6428,12 @@ int tracing_set_tracer(struct trace_array *tr, cons= t char *buf) if (tr->current_trace->reset) tr->current_trace->reset(tr); =20 +#ifdef CONFIG_TRACER_MAX_TRACE + had_max_tr =3D tr->current_trace->use_max_tr; + /* Current trace needs to be nop_trace before synchronize_rcu */ tr->current_trace =3D &nop_trace; =20 -#ifdef CONFIG_TRACER_MAX_TRACE - had_max_tr =3D tr->allocated_snapshot; - if (had_max_tr && !t->use_max_tr) { /* * We need to make sure that the update_max_tr sees that @@ -6446,11 +6446,13 @@ int tracing_set_tracer(struct trace_array *tr, cons= t char *buf) free_snapshot(tr); } =20 - if (t->use_max_tr && !had_max_tr) { + if (t->use_max_tr && !tr->allocated_snapshot) { ret =3D tracing_alloc_snapshot_instance(tr); if (ret < 0) goto out; } +#else + tr->current_trace =3D &nop_trace; #endif =20 if (t->init) { --=20 2.35.1