On 2020/4/13 23:02, Steven Rostedt wrote:

On Mon, 13 Apr 2020 15:12:52 +0800 Xiao Yangyangx.jy@cn.fujitsu.com wrote:

...

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 Yangyangx.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; }

Why register if the allocation failed? Just switch the logic:

int ret = tracing_alloc_snapshot_instance(file->tr);

if (ret != 0) return 0;

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

Hi Steve,

It looks simpler and better. I think we can drop the unnecessary ret variable as well.

Thanks, Xiao Yang

-- Steve

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