perf/core: Change to POLLERR for pinned events with error

Commit:

  f4b07fd ("perf/core: Use POLLHUP for pinned events in error")

started to emit POLLHUP for pinned events in an error state.

But the POLLHUP is also used to signal events that the attached task is
terminated.  To distinguish pinned per-task events in the error state
it would need to check if the task is live.

Change it to POLLERR to make it clear.

Suggested-by: Gabriel Marin <[email protected]>
Signed-off-by: Namhyung Kim <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: namhyung

kernel/events/core.c

@@ -3943,7 +3943,7 @@ static int merge_sched_in(struct perf_event *event, void *data)
 			perf_event_set_state(event, PERF_EVENT_STATE_ERROR);
 
 			if (*perf_event_fasync(event))
-				event->pending_kill = POLL_HUP;
+				event->pending_kill = POLL_ERR;
 
 			perf_event_wakeup(event);
 		} else {
@@ -6075,7 +6075,7 @@ static __poll_t perf_poll(struct file *file, poll_table *wait)
 
 	if (unlikely(READ_ONCE(event->state) == PERF_EVENT_STATE_ERROR &&
 		     event->attr.pinned))
-		return events;
+		return EPOLLERR;
 
 	/*
 	 * Pin the event->rb by taking event->mmap_mutex; otherwise