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

[Steven Rostedt <rostedt@goodmis.org>]

On Mon, 13 Apr 2020 15:12:52 +0800
Xiao Yang <yangx.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 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;
>  	}


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);


-- Steve