Re: [PATCH 1/1] tracing: fix a WARN from trace_event_dyn_put_ref

Krister Johansen
2022-08-16T20:53:08Z

On Tue, Aug 16, 2022 at 03:24:11PM -0400, Steven Rostedt wrote:
> Actually, I think the issue is that the unreg() calls trace_event_put_ref()
> but the reg() does not do the try_get_ref(), but the perf_trace_init()
> does. Which is the broken symmetry.
>
> I think if we pull out the trace_event_put_ref() from the unreg() function,
> we fix the bug and also put back the symmetry.
>
> Does this patch work?

Yes, thanks, this change looks correct. It also does build and pass the
tests that I included in the original e-mail.

> diff --git a/kernel/trace/trace_event_perf.c b/kernel/trace/trace_event_perf.c
> index a114549720d6..61e3a2620fa3 100644
> --- a/kernel/trace/trace_event_perf.c
> +++ b/kernel/trace/trace_event_perf.c
> @@ -157,7 +157,7 @@ static void perf_trace_event_unreg(struct perf_event *p_event)
> int i;
>
> if (--tp_event->perf_refcount > 0)
> - goto out;
> + return;
>
> tp_event->class->reg(tp_event, TRACE_REG_PERF_UNREGISTER, NULL);
>
> @@ -176,8 +176,6 @@ static void perf_trace_event_unreg(struct perf_event *p_event)
> perf_trace_buf[i] = NULL;
> }
> }
> -out:
> - trace_event_put_ref(tp_event);
> }
>
> static int perf_trace_event_open(struct perf_event *p_event)
> @@ -241,6 +239,7 @@ void perf_trace_destroy(struct perf_event *p_event)
> mutex_lock(&event_mutex);
> perf_trace_event_close(p_event);
> perf_trace_event_unreg(p_event);
> + trace_event_put_ref(p_event->tp_event);
> mutex_unlock(&event_mutex);
> }
>
> @@ -292,6 +291,7 @@ void perf_kprobe_destroy(struct perf_event *p_event)
> mutex_lock(&event_mutex);
> perf_trace_event_close(p_event);
> perf_trace_event_unreg(p_event);
> + trace_event_put_ref(p_event->tp_event);
> mutex_unlock(&event_mutex);
>
> destroy_local_trace_kprobe(p_event->tp_event);
> @@ -347,6 +347,7 @@ void perf_uprobe_destroy(struct perf_event *p_event)
> mutex_lock(&event_mutex);
> perf_trace_event_close(p_event);
> perf_trace_event_unreg(p_event);
> + trace_event_put_ref(p_event->tp_event);
> mutex_unlock(&event_mutex);
> destroy_local_trace_uprobe(p_event->tp_event);
> }
>

If it matters:

Reviewed-by: Krister Johansen <kjlx@templeofstupid.com>
Tested-by: Krister Johansen <kjlx@templeofstupid.com>

Thanks again,

-K