[PATCH v7 01/14] alpha/perf: do not update an event that has no counter in alpha_pmu_del()

[Matt Turner <mattst88@gmail.com>]

alpha_pmu_add() puts a new event on the list with current_idx set to
PMC_NO_INDEX, and alpha_pmu_enable() assigns the counter later, through
maybe_change_configuration(). Alpha has no group transaction callbacks,
so the core schedules a group with the PMU disabled for the whole
group. When a sibling does not fit, for example because the counters
are multiplexed between groups, group_sched_in() rolls back the events
it already added by calling alpha_pmu_del() on them, before the PMU is
enabled again.

alpha_pmu_del() then passes idx == -1 to alpha_perf_event_update(),
which indexes pmc_count_shift[], pmc_count_mask[] and pmc_max_period[]
at -1, and it clears bit -1 of idx_mask, an undefined shift. On the
EV67 PMU the entries before each array happen to make the computed
delta zero, so the event's count survives, but only by luck of the
struct layout.

The event was never on a counter, so there is no count to absorb and no
counter to turn off. Only remove it from the list.

Fixes: 979f867191f8 ("alpha: implement HW performance events on the EV67 and later CPUs")
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Matt Turner <mattst88@gmail.com>
---
 arch/alpha/kernel/perf_event.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/arch/alpha/kernel/perf_event.c b/arch/alpha/kernel/perf_event.c
index a3eaab094ece..f77abeeb25b8 100644
--- a/arch/alpha/kernel/perf_event.c
+++ b/arch/alpha/kernel/perf_event.c
@@ -506,11 +506,17 @@ static void alpha_pmu_del(struct perf_event *event, int flags)
 					cpuc->current_idx[j];
 			}
 
-			/* Absorb the final count and turn off the event. */
-			alpha_perf_event_update(event, hwc, idx, 0);
-			perf_event_update_userpage(event);
+			/*
+			 * Absorb the final count and turn off the event,
+			 * unless a failed group schedule rolled it back
+			 * before it was given a counter.
+			 */
+			if (idx != PMC_NO_INDEX) {
+				alpha_perf_event_update(event, hwc, idx, 0);
+				perf_event_update_userpage(event);
 
-			cpuc->idx_mask &= ~(1UL<<idx);
+				cpuc->idx_mask &= ~(1UL<<idx);
+			}
 			cpuc->n_events--;
 			break;
 		}

-- 
2.55.0