Re: [PATCH] perf/core: Fix the same task check in perf_event_set_output

[Adrian Hunter <adrian.hunter@intel.com>]

On 11/07/22 21:07, kan.liang@linux.intel.com wrote:
> From: Kan Liang <kan.liang@linux.intel.com>
> 
> With the --per-thread option, perf record errors out when sampling with
> a hardware event and a software event as below.
> 
>  $ perf record -e cycles,dummy --per-thread ls
>  failed to mmap with 22 (Invalid argument)
> 
> The same task is sampled with the two events. The IOC_OUTPUT is invoked
> to share the mmap memory of the task between the events. In the
> perf_event_set_output(), the event->ctx is used to check whether the
> two events are attached to the same task. However, a hardware event and
> a software event are from different task context. The check always
> fails.
> 
> The task struct is stored in the event->hw.target for each per-thread
> event. It can be used to determine whether two events are attached to
> the same task.
> 
> The patch can also fix another issue reported months ago.
> https://lore.kernel.org/all/92645262-D319-4068-9C44-2409EF44888E@gmail.com/
> The event->ctx is not ready when the perf_event_set_output() is invoked
> in the perf_event_open(), while the event->hw.target has been assigned
> at the moment.
> 
> The problem should be a long time issue since commit c3f00c70276d
> ("perf: Separate find_get_context() from event initialization"). The
> event->hw.target doesn't exist at that time. Here, the patch which
> introduces the event->hw.target is used by the Fixes tag.
> 
> The problem should still exists between the broken patch and the
> event->hw.target patch. This patch does not intend to fix that case.
> 
> Fixes: 50f16a8bf9d7 ("perf: Remove type specific target pointers")
> Reviewed-by: Zhengjun Xing <zhengjun.xing@linux.intel.com>
> Signed-off-by: Kan Liang <kan.liang@linux.intel.com>

Did this slip through the cracks, or is there more complexity
to this case than just sharing the rb?

> ---
>  kernel/events/core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index b4d62210c3e5..22df79d3f19d 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -12080,7 +12080,7 @@ perf_event_set_output(struct perf_event *event, struct perf_event *output_event)
>  	/*
>  	 * If its not a per-cpu rb, it must be the same task.
>  	 */
> -	if (output_event->cpu == -1 && output_event->ctx != event->ctx)
> +	if (output_event->cpu == -1 && output_event->hw.target != event->hw.target)
>  		goto out;
>  
>  	/*