Messages in this thread | | | Date | Tue, 14 Apr 2020 09:59:48 +0800 | From | Xiao Yang <> | Subject | Re: [PATCH] tracing: Fix the race between registering 'snapshot' event trigger and triggering 'snapshot' operation |
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On 2020/4/13 23:02, Steven Rostedt wrote: > 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); 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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