[tip: perf/core] perf/hw_breakpoint: Annotate tsk->perf_event_mutex vs ctx->mutex

[tip-bot2 for Peter Zijlstra]

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

Commit-ID:     82aad7ff7ac25c8cf09d491ae23b9823f1901486
Gitweb:        https://git.kernel.org/tip/82aad7ff7ac25c8cf09d491ae23b9823f1901486
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Tue, 04 Oct 2022 12:20:39 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Tue, 04 Oct 2022 13:32:09 +02:00

perf/hw_breakpoint: Annotate tsk->perf_event_mutex vs ctx->mutex

Perf fuzzer gifted a lockdep splat:

  perf_event_init_context()
    mutex_lock(parent_ctx->mutex);			(B)
    inherit_task_group()
      inherit_group()
        inherit_event()
          perf_event_alloc()
            perf_try_init_event() := hw_breakpoint_event_init()
              register_perf_hw_breakpoint()
                mutex_lock(child->perf_event_mutex);	(A)

Which is against the normal (documented) order. Now, this is a false
positive in that child is not published yet, but also inherited events
never end up on ->perf_event_list.

Annotate this one away.

Fixes: 0912037fec11 ("perf/hw_breakpoint: Reduce contention with large number of tasks")
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 kernel/events/hw_breakpoint.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
index 7ef0e98..c379770 100644
--- a/kernel/events/hw_breakpoint.c
+++ b/kernel/events/hw_breakpoint.c
@@ -117,7 +117,17 @@ static struct mutex *bp_constraints_lock(struct perf_event *bp)
 	struct mutex *tsk_mtx = get_task_bps_mutex(bp);
 
 	if (tsk_mtx) {
-		mutex_lock(tsk_mtx);
+		/*
+		 * Fully analogous to the perf_try_init_event() nesting
+		 * argument in the comment near perf_event_ctx_lock_nested();
+		 * this child->perf_event_mutex cannot ever deadlock against
+		 * the parent->perf_event_mutex usage from
+		 * perf_event_task_{en,dis}able().
+		 *
+		 * Specifically, inherited events will never occur on
+		 * ->perf_event_list.
+		 */
+		mutex_lock_nested(tsk_mtx, SINGLE_DEPTH_NESTING);
 		percpu_down_read(&bp_cpuinfo_sem);
 	} else {
 		percpu_down_write(&bp_cpuinfo_sem);