[PATCH] tracing: Fix the race between registering 'snapshot' event trigger and triggering 'snapshot' operation

[Xiao Yang <yangx.jy@cn.fujitsu.com>]

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 spare buffer for 'snapshot' event trigger.
'snapshot' operation always detects the lack of allocated buffer in the rare
case so make register_snapshot_trigger() allocate spare 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!   ***
-----------------------------------------------------------

Signed-off-by: Xiao Yang <yangx.jy@cn.fujitsu.com>
---
 kernel/trace/trace_events_trigger.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/trace_events_trigger.c b/kernel/trace/trace_events_trigger.c
index dd34a1b46a86..00e54cdcef3e 100644
--- a/kernel/trace/trace_events_trigger.c
+++ b/kernel/trace/trace_events_trigger.c
@@ -1088,9 +1088,13 @@ register_snapshot_trigger(char *glob, struct event_trigger_ops *ops,
 			  struct event_trigger_data *data,
 			  struct trace_event_file *file)
 {
-	int ret = register_trigger(glob, ops, data, file);
+	int alloc_ret, ret;
 
-	if (ret > 0 && tracing_alloc_snapshot_instance(file->tr) != 0) {
+	alloc_ret = tracing_alloc_snapshot_instance(file->tr);
+
+	ret = register_trigger(glob, ops, data, file);
+
+	if (ret > 0 && alloc_ret != 0) {
 		unregister_trigger(glob, ops, data, file);
 		ret = 0;
 	}
-- 
2.25.1