[PATCH 4.9 084/125] tracing: Fix the race between registering snapshot event trigger and triggering snapshot operation

[Greg Kroah-Hartman]

From: Xiao Yang <yangx.jy@xxxxxxxxxxxxxx>

commit 0bbe7f719985efd9adb3454679ecef0984cb6800 upstream.

Traced event can trigger 'snapshot' operation(i.e. calls snapshot_trigger()
or snapshot_count_trigger()) when register_snapshot_trigger() has completed
registration but doesn't allocate buffer for 'snapshot' event trigger.  In
the rare case, 'snapshot' operation always detects the lack of allocated
buffer so make register_snapshot_trigger() allocate buffer first.

trigger-snapshot.tc in kselftest reproduces the issue on slow vm:
-----------------------------------------------------------
cat trace
...
ftracetest-3028  [002] ....   236.784290: sched_process_fork: comm=ftracetest pid=3028 child_comm=ftracetest child_pid=3036
     <...>-2875  [003] ....   240.460335: tracing_snapshot_instance_cond: *** SNAPSHOT NOT ALLOCATED ***
     <...>-2875  [003] ....   240.460338: tracing_snapshot_instance_cond: *** stopping trace here!   ***
-----------------------------------------------------------

Link: http://lkml.kernel.org/r/20200414015145.66236-1-yangx.jy@xxxxxxxxxxxxxx

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 93e31ffbf417a ("tracing: Add 'snapshot' event trigger command")
Signed-off-by: Xiao Yang <yangx.jy@xxxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 kernel/trace/trace_events_trigger.c |   10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

--- a/kernel/trace/trace_events_trigger.c
+++ b/kernel/trace/trace_events_trigger.c
@@ -1068,14 +1068,10 @@ register_snapshot_trigger(char *glob, st
 			  struct event_trigger_data *data,
 			  struct trace_event_file *file)
 {
-	int ret = register_trigger(glob, ops, data, file);
+	if (tracing_alloc_snapshot() != 0)
+		return 0;
 
-	if (ret > 0 && tracing_alloc_snapshot() != 0) {
-		unregister_trigger(glob, ops, data, file);
-		ret = 0;
-	}
-
-	return ret;
+	return register_trigger(glob, ops, data, file);
 }
 
 static int