perf/hw_breakpoint: Provide hw_breakpoint_is_used() and use in test

Provide hw_breakpoint_is_used() to check if breakpoints are in use on
the system.

Use it in the KUnit test to verify the global state before and after a
test case.

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Acked-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20220829124719.675715-3-elver@google.com

include/linux/hw_breakpoint.h

@@ -74,6 +74,7 @@ register_wide_hw_breakpoint(struct perf_event_attr *attr,
 extern int register_perf_hw_breakpoint(struct perf_event *bp);
 extern void unregister_hw_breakpoint(struct perf_event *bp);
 extern void unregister_wide_hw_breakpoint(struct perf_event * __percpu *cpu_events);
+extern bool hw_breakpoint_is_used(void);
 
 extern int dbg_reserve_bp_slot(struct perf_event *bp);
 extern int dbg_release_bp_slot(struct perf_event *bp);
@@ -121,6 +122,8 @@ register_perf_hw_breakpoint(struct perf_event *bp)	{ return -ENOSYS; }
 static inline void unregister_hw_breakpoint(struct perf_event *bp)	{ }
 static inline void
 unregister_wide_hw_breakpoint(struct perf_event * __percpu *cpu_events)	{ }
+static inline bool hw_breakpoint_is_used(void)		{ return false; }
+
 static inline int
 reserve_bp_slot(struct perf_event *bp)			{return -ENOSYS; }
 static inline void release_bp_slot(struct perf_event *bp) 		{ }

kernel/events/hw_breakpoint.c

@@ -604,6 +604,35 @@ void unregister_wide_hw_breakpoint(struct perf_event * __percpu *cpu_events)
 }
 EXPORT_SYMBOL_GPL(unregister_wide_hw_breakpoint);
 
+/**
+ * hw_breakpoint_is_used - check if breakpoints are currently used
+ *
+ * Returns: true if breakpoints are used, false otherwise.
+ */
+bool hw_breakpoint_is_used(void)
+{
+	int cpu;
+
+	if (!constraints_initialized)
+		return false;
+
+	for_each_possible_cpu(cpu) {
+		for (int type = 0; type < TYPE_MAX; ++type) {
+			struct bp_cpuinfo *info = get_bp_info(cpu, type);
+
+			if (info->cpu_pinned)
+				return true;
+
+			for (int slot = 0; slot < nr_slots[type]; ++slot) {
+				if (info->tsk_pinned[slot])
+					return true;
+			}
+		}
+	}
+
+	return false;
+}
+
 static struct notifier_block hw_breakpoint_exceptions_nb = {
 	.notifier_call = hw_breakpoint_exceptions_notify,
 	/* we need to be notified first */

kernel/events/hw_breakpoint_test.c

@@ -294,7 +294,14 @@ static struct kunit_case hw_breakpoint_test_cases[] = {
 static int test_init(struct kunit *test)
 {
 	/* Most test cases want 2 distinct CPUs. */
-	return num_online_cpus() < 2 ? -EINVAL : 0;
+	if (num_online_cpus() < 2)
+		return -EINVAL;
+
+	/* Want the system to not use breakpoints elsewhere. */
+	if (hw_breakpoint_is_used())
+		return -EBUSY;
+
+	return 0;
 }
 
 static void test_exit(struct kunit *test)
@@ -308,6 +315,9 @@ static void test_exit(struct kunit *test)
 		kthread_stop(__other_task);
 		__other_task = NULL;
 	}
+
+	/* Verify that internal state agrees that no breakpoints are in use. */
+	KUNIT_EXPECT_FALSE(test, hw_breakpoint_is_used());
 }
 
 static struct kunit_suite hw_breakpoint_test_suite = {