Hi Peter,
On Wed, Jul 5, 2023 at 1:38 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> 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?
Yep, looks good. Do you want me to resend?
Thanks,
Namhyung
>
> > + 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
> >
