perf: Clean up mediated vPMU accounting

The mediated_pmu_account_event() and perf_create_mediated_pmu()
functions implement the exclusion between '!exclude_guest' counters
and mediated vPMUs. Their implementation is basically identical,
except mirrored in what they count/check.

Make sure the actual implementations reflect this similarity.

Notably:
 - while perf_release_mediated_pmu() has an underflow check;
   mediated_pmu_unaccount_event() did not.
 - while perf_create_mediated_pmu() has an inc_not_zero() path;
   mediated_pmu_account_event() did not.

Also, the inc_not_zero() path can be outsite of
perf_mediated_pmu_mutex. The mutex must guard the 0->1 (of either
nr_include_guest_events or nr_mediated_pmu_vms) transition, but once a
counter is already non-zero, it can safely be incremented further.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251208115156.GE3707891@noisy.programming.kicks-ass.net

Author: Peter Zijlstra

kernel/events/core.c

@@ -6344,8 +6344,10 @@ static int mediated_pmu_account_event(struct perf_event *event)
 	if (!is_include_guest_event(event))
 		return 0;
 
-	guard(mutex)(&perf_mediated_pmu_mutex);
+	if (atomic_inc_not_zero(&nr_include_guest_events))
+		return 0;
 
+	guard(mutex)(&perf_mediated_pmu_mutex);
 	if (atomic_read(&nr_mediated_pmu_vms))
 		return -EOPNOTSUPP;
 
@@ -6358,6 +6360,9 @@ static void mediated_pmu_unaccount_event(struct perf_event *event)
 	if (!is_include_guest_event(event))
 		return;
 
+	if (WARN_ON_ONCE(!atomic_read(&nr_include_guest_events)))
+		return;
+
 	atomic_dec(&nr_include_guest_events);
 }
 
@@ -6373,10 +6378,10 @@ static void mediated_pmu_unaccount_event(struct perf_event *event)
  */
 int perf_create_mediated_pmu(void)
 {
-	guard(mutex)(&perf_mediated_pmu_mutex);
 	if (atomic_inc_not_zero(&nr_mediated_pmu_vms))
 		return 0;
 
+	guard(mutex)(&perf_mediated_pmu_mutex);
 	if (atomic_read(&nr_include_guest_events))
 		return -EBUSY;