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@infradead.org>

diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
index 7ef0e98d31e28e3f451744d503fec9ca5f5a5a5b..c3797701339cbd575ca3c58ed9f0313e16e74cc8 100644 (file)
--- 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);