Commit 57f2a2ad authored Apr 14, 2020 by Xiao Yang Committed by Greg Kroah-Hartman Apr 21, 2020

tracing: Fix the race between registering 'snapshot' event trigger and...

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

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@cn.fujitsu.com



Cc: stable@vger.kernel.org
Fixes: 93e31ffb ("tracing: Add 'snapshot' event trigger command")
Signed-off-by: Xiao Yang <yangx.jy@cn.fujitsu.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

parent 0aadc67c

kernel/trace/trace_events_trigger.c

+3 −7

Original line number	Diff line number	Diff line
		@@ -1081,14 +1081,10 @@ 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);

		if (ret > 0 && tracing_alloc_snapshot_instance(file->tr) != 0) {
		unregister_trigger(glob, ops, data, file);
		ret = 0;
		}
		if (tracing_alloc_snapshot_instance(file->tr) != 0)
		return 0;

		return ret;
		return register_trigger(glob, ops, data, file);
		}

		static int