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@google.com Cc: stable@vger.kernel.org Signed-off-by: Namhyung Kim namhyung@kernel.org

---

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

The following commit has been merged into the perf/urgent branch of tip:

Commit-ID: 27c68c216ee1f1b086e789a64486e6511e380b8a Gitweb: https://git.kernel.org/tip/27c68c216ee1f1b086e789a64486e6511e380b8a Author: Namhyung Kim namhyung@kernel.org AuthorDate: Tue, 04 Jul 2023 11:15:15 -07:00 Committer: Peter Zijlstra peterz@infradead.org CommitterDate: Mon, 10 Jul 2023 09:52:20 +02:00

perf/x86: Fix lockdep warning in for_each_sibling_event() on SPR

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@google.com Signed-off-by: Namhyung Kim namhyung@kernel.org Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Cc: stable@vger.kernel.org Link: https://lkml.kernel.org/r/20230704181516.3293665-1-namhyung@kernel.org --- arch/x86/events/intel/core.c | 7 +++++++ 1 file changed, 7 insertions(+)

diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c index a149faf..2a284ba 100644 --- a/arch/x86/events/intel/core.c +++ b/arch/x86/events/intel/core.c @@ -3993,6 +3993,13 @@ static int intel_pmu_hw_config(struct perf_event *event) struct perf_event *leader = event->group_leader; struct perf_event *sibling = NULL;

+ /* + * When this memload event is also the first event (no group + * exists yet), then there is no aux event before it. + */ + 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))