perf/arm-smmuv3: Fix lockdep assert in ->event_init()

Same as
https://lore.kernel.org/all/20240514180050.182454-1-namhyung@kernel.org/,
we should skip `for_each_sibling_event()` for group leader since it
doesn't have the ctx yet.

Fixes: f3c0eba ("perf: Add a few assertions")
Reported-by: Greg Thelen <gthelen@google.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: Tuan Phan <tuanphan@os.amperecomputing.com>
Signed-off-by: Chun-Tse Shao <ctshao@google.com>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20241108050806.3730811-1-ctshao@google.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: Chun-Tse Shao

drivers/perf/arm_smmuv3_pmu.c

@@ -431,6 +431,17 @@ static int smmu_pmu_event_init(struct perf_event *event)
 			return -EINVAL;
 	}
 
+	/*
+	 * Ensure all events are on the same cpu so all events are in the
+	 * same cpu context, to avoid races on pmu_enable etc.
+	 */
+	event->cpu = smmu_pmu->on_cpu;
+
+	hwc->idx = -1;
+
+	if (event->group_leader == event)
+		return 0;
+
 	for_each_sibling_event(sibling, event->group_leader) {
 		if (is_software_event(sibling))
 			continue;
@@ -442,14 +453,6 @@ static int smmu_pmu_event_init(struct perf_event *event)
 			return -EINVAL;
 	}
 
-	hwc->idx = -1;
-
-	/*
-	 * Ensure all events are on the same cpu so all events are in the
-	 * same cpu context, to avoid races on pmu_enable etc.
-	 */
-	event->cpu = smmu_pmu->on_cpu;
-
 	return 0;
 }