linux-kernel - [PATCH 2/2 v2] tracing: Fix ftrace_boot

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Date:   Tue, 04 Apr 2023 22:21:15 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ross Zwisler <zwisler@...gle.com>, stable@...r.kernel.org
Subject: [PATCH 2/2 v2] tracing: Fix ftrace_boot_snapshot command line logic

From: "Steven Rostedt (Google)" <rostedt@...dmis.org>

The kernel command line ftrace_boot_snapshot by itself is supposed to
trigger a snapshot at the end of boot up of the main top level trace
buffer. A ftrace_boot_snapshot=foo will do the same for an instance called
foo that was created by trace_instance=foo,...

The logic was broken where if ftrace_boot_snapshot was by itself, it would
trigger a snapshot for all instances that had tracing enabled, regardless
if it asked for a snapshot or not.

When a snapshot is requested for a buffer, the buffer's
tr->allocated_snapshot is set to true. Use that to know if a trace buffer
wants a snapshot at boot up or not.

Since the top level buffer is part of the ftrace_trace_arrays list,
there's no reason to treat it differently than the other buffers. Just
iterate the list if ftrace_boot_snapshot was specified.

Cc: stable@...r.kernel.org
Fixes: 9c1c251d670bc ("tracing: Allow boot instances to have snapshot buffers")
Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
---
Changes since v1: https://lkml.kernel.org/r/20230404230308.501833715@goodmis.org

 - Protect use of tr->allocated_snapshot around #ifdef TRACER_MAX_TRACE

 kernel/trace/trace.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 93740a9370c6..36a6037823cd 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -10394,19 +10394,20 @@ __init static int tracer_alloc_buffers(void)

 void __init ftrace_boot_snapshot(void)
 {
+#ifdef CONFIG_TRACER_MAX_TRACE
 	struct trace_array *tr;

-	if (snapshot_at_boot) {
-		tracing_snapshot();
-		internal_trace_puts("** Boot snapshot taken **\n");
-	}
+	if (!snapshot_at_boot)
+		return;

 	list_for_each_entry(tr, &ftrace_trace_arrays, list) {
-		if (tr == &global_trace)
+		if (!tr->allocated_snapshot)
 			continue;
-		trace_array_puts(tr, "** Boot snapshot taken **\n");
+
 		tracing_snapshot_instance(tr);
+		trace_array_puts(tr, "** Boot snapshot taken **\n");
 	}
+#endif
 }

 void __init early_trace_init(void)
-- 
2.39.2