[PATCH v2] tracing: Do not free snapshot if tracer is on cmdline

[Steven Rostedt]

From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx>

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 <mhiramat@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Fixes: 45ad21ca5530 ("tracing: Have trace_array keep track if snapshot buffer is allocated")
Reported-by: Ross Zwisler <zwisler@xxxxxxxxxx>
Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
---
Changes since v1: https://lkml.kernel.org/r/20221004180452.6cf967f8@xxxxxxxxxxxxxxxxxx
  - 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, const char *buf)
 	if (tr->current_trace->reset)
 		tr->current_trace->reset(tr);
 
+#ifdef CONFIG_TRACER_MAX_TRACE
+	had_max_tr = tr->current_trace->use_max_tr;
+
 	/* Current trace needs to be nop_trace before synchronize_rcu */
 	tr->current_trace = &nop_trace;
 
-#ifdef CONFIG_TRACER_MAX_TRACE
-	had_max_tr = 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, const char *buf)
 		free_snapshot(tr);
 	}
 
-	if (t->use_max_tr && !had_max_tr) {
+	if (t->use_max_tr && !tr->allocated_snapshot) {
 		ret = tracing_alloc_snapshot_instance(tr);
 		if (ret < 0)
 			goto out;
 	}
+#else
+	tr->current_trace = &nop_trace;
 #endif
 
 	if (t->init) {
-- 
2.35.1