Re: [PATCH] perf/arm-dmc620: Fix lockdep assert in ->event_init()

From: Robin Murphy
Date: Wed May 15 2024 - 08:12:14 EST


On 2024-05-14 7:00 pm, Namhyung Kim wrote:
for_each_sibling_event() checks leader's ctx but it doesn't have the ctx
yet if it's the leader. Like in perf_event_validate_size(), we should
skip checking siblings in that case.

Ugh, looking around for_each_sibling_event() sites, it looks like there are a fair few other drivers using this pattern as well :(

I'd love for groups to be less horribly complicated, but I think I can follow the underlying reasoning here. I suppose one could argue that the assertion could take into account that there's nothing to protect in the case where event->ctx is still NULL, since nobody else should be able to touch the event's own empty sibling list at this point before perf_event_open() has even returned. However by the same token there's also no real reason for drivers *not* to return early when they equally can tell that the sibling list must be empty, and indeed that seems to be a fairly common pattern too, so I see no issue with fixing up all the offending drivers for consistency either.

Reviewed-by: Robin Murphy <robin.murphy@xxxxxxx>

Fixes: f3c0eba287049 ("perf: Add a few assertions")
Reported-by: Greg Thelen <gthelen@xxxxxxxxxx>
Cc: Robin Murphy <robin.murphy@xxxxxxx>
Cc: Tuan Phan <tuanphan@xxxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
---
drivers/perf/arm_dmc620_pmu.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/perf/arm_dmc620_pmu.c b/drivers/perf/arm_dmc620_pmu.c
index 8a81be2dd5ec..88c17c1d6d49 100644
--- a/drivers/perf/arm_dmc620_pmu.c
+++ b/drivers/perf/arm_dmc620_pmu.c
@@ -542,12 +542,16 @@ static int dmc620_pmu_event_init(struct perf_event *event)
if (event->cpu < 0)
return -EINVAL;
+ hwc->idx = -1;
+
+ if (event->group_leader == event)
+ return 0;
+
/*
* We can't atomically disable all HW counters so only one event allowed,
* although software events are acceptable.
*/
- if (event->group_leader != event &&
- !is_software_event(event->group_leader))
+ if (!is_software_event(event->group_leader))
return -EINVAL;
for_each_sibling_event(sibling, event->group_leader) {
@@ -556,7 +560,6 @@ static int dmc620_pmu_event_init(struct perf_event *event)
return -EINVAL;
}
- hwc->idx = -1;
return 0;
}