Re: [PATCH v4] perf fdarray: Fix destructor invocation and event counting in fdarray__filter

[Namhyung Kim <namhyung@kernel.org>]

On Mon, Aug 17, 2026 at 04:04:31PM -0700, Ian Rogers wrote:
> When processing POLLHUP or POLLERR for an event in fdarray__filter, the
> function invokes its destructor callback. However, the exact behavior
> around unhandled POLLHUP events on control pipe descriptors caused
> premature termination due to thread and evlist pollfd index mismatches.
> This occurred because they were skipped by the early nonfilterable continue.

Do you have a concrete scenario to check this behavior so that we can
verify the fix?  It'd be great if we can add a test case.

Thanks,
Namhyung

> 
> Address this by refining the early continue filter to only skip system-wide
> perf events (which are nonfilterable but not non_perf_event). Control
> descriptors (non_perf_event) now fall through to appropriately have their
> fd value unset to -1 on POLLHUP while avoiding their destructors. Finally,
> maintain the invariant that the active event counter (nr) increments strictly
> and only for completely filterable events.
> 
> Fixes: fb4751e79c45 ("perf record: Fix teardown hang on system-wide multi-threaded sessions")
> Assisted-by: Gemini:gemini-3.1-pro
> Signed-off-by: Ian Rogers <irogers@google.com>
> ---
>  tools/lib/api/fd/array.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/lib/api/fd/array.c b/tools/lib/api/fd/array.c
> index 67b73481df27..3681ad7c6527 100644
> --- a/tools/lib/api/fd/array.c
> +++ b/tools/lib/api/fd/array.c
> @@ -116,14 +116,23 @@ int fdarray__filter(struct fdarray *fda, short revents,
>  		return 0;
>  
>  	for (fd = 0; fd < fda->nr; ++fd) {
> -		if (fda->priv[fd].flags & fdarray_flag__nonfilterable)
> +		/*
> +		 * System-wide perf events are nonfilterable but not non_perf_event.
> +		 * We want to skip them entirely and never process revents on them.
> +		 */
> +		if ((fda->priv[fd].flags & fdarray_flag__nonfilterable) &&
> +		    !(fda->priv[fd].flags & fdarray_flag__non_perf_event))
>  			continue;
>  
>  		if (!fda->entries[fd].events)
>  			continue;
>  
>  		if (fda->entries[fd].revents & revents) {
> -			if (entry_destructor)
> +			/*
> +			 * Control descriptors are non_perf_event and don't need
> +			 * their perf-specific destructors triggered.
> +			 */
> +			if (entry_destructor && !(fda->priv[fd].flags & fdarray_flag__non_perf_event))
>  				entry_destructor(fda, fd, arg);
>  
>  			/*
> @@ -136,7 +145,8 @@ int fdarray__filter(struct fdarray *fda, short revents,
>  			continue;
>  		}
>  
> -		++nr;
> +		if (!(fda->priv[fd].flags & fdarray_flag__nonfilterable))
> +			++nr;
>  	}
>  
>  	return nr;
> -- 
> 2.55.0.699.gb54405d56f-goog
>