tracing: Fix snapshot counter going between two tracers that use it

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.

Link: https://lore.kernel.org/linux-trace-kernel/20240223013344.570525723@goodmis.org
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Vincent Donnefort <vdonnefort@google.com>
Fixes: 16f7e48ffc53a ("tracing: Add snapshot refcount")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index a7c0dc0aaf147e959e3557e21c1bd99cbaf39c5c..1e1fd377a1cfdd6e532c3155055c28f9aaf86fbd 100644 (file)
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -6162,7 +6162,7 @@ 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;