linux-kernel - [PATCH 1/2] tracing: Fix snapshot counter going between two tracers that use it

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Date: Thu, 22 Feb 2024 20:33:24 -0500
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>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Vincent Donnefort <vdonnefort@...gle.com>
Subject: [PATCH 1/2] tracing: Fix snapshot counter going between two tracers that use it

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

Running the ftrace selftests caused the ring buffer mapping test to fail.
Investigating, I found that the snapshot counter would be incremented
every time a tracer that uses the snapshot is enabled even if the snapshot
was used by the previous tracer.

That is:

 # cd /sys/kernel/tracing
 # echo wakeup_rt > current_tracer
 # echo wakeup_dl > current_tracer
 # echo nop > current_tracer

would leave the snapshot counter at 1 and not zero. That's because the
enabling of wakeup_dl would increment the counter again but the setting
the tracer to nop would only decrement it once.

Do not arm the snapshot for a tracer if the previous tracer already had it
armed.

Fixes: 16f7e48ffc53a ("tracing: Add snapshot refcount")
Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
---
 kernel/trace/trace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index b7a870c8ae2a..480201c3b36e 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -6164,7 +6164,7 @@ int tracing_set_tracer(struct trace_array *tr, const char *buf)
 		tracing_disarm_snapshot(tr);
 	}
 
-	if (t->use_max_tr) {
+	if (!had_max_tr && t->use_max_tr) {
 		ret = tracing_arm_snapshot_locked(tr);
 		if (ret)
 			goto out;
-- 
2.43.0