Re: [PATCH] perf/x86: Fix lockdep warning in for_each_sibling_event() on SPR

[Namhyung Kim]

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
> >