On Tue, Jul 04, 2023 at 11:15:15AM -0700, Namhyung Kim wrote:
> On SPR, the load latency event needs an auxiliary event in the same
> group to work properly. There's a check in intel_pmu_hw_config()
> for this to iterate sibling events and find a mem-loads-aux event.
>
> The for_each_sibling_event() has a lockdep assert to make sure if it
> disabled hardirq or hold leader->ctx->mutex. This works well if the
> given event has a separate leader event since perf_try_init_event()
> grabs the leader->ctx->mutex to protect the sibling list. But it can
> cause a problem when the event itself is a leader since the event is
> not initialized yet and there's no ctx for the event.
>
> Actually I got a lockdep warning when I run the below command on SPR,
> but I guess it could be a NULL pointer dereference.
>
> $ perf record -d -e cpu/mem-loads/uP true
>
> The code path to the warning is:
>
> sys_perf_event_open()
> perf_event_alloc()
> perf_init_event()
> perf_try_init_event()
> x86_pmu_event_init()
> hsw_hw_config()
> intel_pmu_hw_config()
> for_each_sibling_event()
> lockdep_assert_event_ctx()
>
> We don't need for_each_sibling_event() when it's a standalone event.
> Let's return the error code directly.
>
> Fixes: f3c0eba28704 ("perf: Add a few assertions")
> Reported-by: Greg Thelen <gthelen@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> ---
> arch/x86/events/intel/core.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> index 0d09245aa8df..933fe4894c32 100644
> --- a/arch/x86/events/intel/core.c
> +++ b/arch/x86/events/intel/core.c
> @@ -3983,6 +3983,14 @@ static int intel_pmu_hw_config(struct perf_event *event)
> struct perf_event *leader = event->group_leader;
> struct perf_event *sibling = NULL;
>
> + /*
> + * The event is not fully initialized yet and no ctx is set
> + * for the event. Avoid for_each_sibling_event() since it
> + * has a lockdep assert with leader->ctx->mutex.
> + */
If I understand things correctly, your patch is indeed correct, however
I don't much like this comment, does something like:
/*
* When this memload event is also the first event (no
* group exists yet), then there is no aux event before
* it.
*/
work for you?
> + if (leader == event)
> + return -ENODATA;
> +
> if (!is_mem_loads_aux_event(leader)) {
> for_each_sibling_event(sibling, leader) {
> if (is_mem_loads_aux_event(sibling))
> --
> 2.41.0.255.g8b1d071c50-goog
>
