tracing: Fix race where histograms can be called before the event

commit 94eedf3 ("tracing: Fix race where eprobes can be called before
the event") fixed an issue where if an event is soft disabled, and the
trigger is being added, there's a small window where the event sees that
there's a trigger but does not see that it requires reading the event yet,
and then calls the trigger with the record == NULL.

This could be solved with adding memory barriers in the hot path, or to
make sure that all the triggers requiring a record check for NULL. The
latter was chosen.

Commit 94eedf3 set the eprobe trigger handle to check for NULL, but
the same needs to be done with histograms.

Link: https://lore.kernel.org/linux-trace-kernel/[email protected]/
Link: https://lore.kernel.org/linux-trace-kernel/[email protected]

Cc: Tom Zanussi <[email protected]>
Cc: [email protected]
Fixes: 7491e2c ("tracing: Add a probe that attaches to trace events")
Reported-by: Masami Hiramatsu (Google) <[email protected]>
Acked-by: Masami Hiramatsu (Google) <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

Author: rostedt

kernel/trace/trace_events_hist.c

@@ -5143,6 +5143,9 @@ static void event_hist_trigger(struct event_trigger_data *data,
 	void *key = NULL;
 	unsigned int i;
 
+	if (unlikely(!rbe))
+		return;
+
 	memset(compound_key, 0, hist_data->key_size);
 
 	for_each_hist_key_field(i, hist_data) {