perf: Add a few assertions

While auditing 6b959ba2 ("perf/core: Fix reentry problem in
perf_output_read_group()") a few spots were found that wanted
assertions.

Notable for_each_sibling_event() relies on exclusion from
modification. This would normally be holding either ctx->lock or
ctx->mutex, however due to how things are constructed disabling IRQs
is a valid and sufficient substitute for ctx->lock.

Another possible site to add assertions would be the various
pmu::{add,del,read,..}() methods, but that's not trivially expressable
in C -- the best option is wrappers, but those are easy enough to
forget.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

include/linux/perf_event.h

@@ -61,6 +61,7 @@ struct perf_guest_info_callbacks {
 #include <linux/refcount.h>
 #include <linux/security.h>
 #include <linux/static_call.h>
+#include <linux/lockdep.h>
 #include <asm/local.h>
 
 struct perf_callchain_entry {
@@ -634,7 +635,23 @@ struct pmu_event_list {
 	struct list_head	list;
 };
 
+/*
+ * event->sibling_list is modified whole holding both ctx->lock and ctx->mutex
+ * as such iteration must hold either lock. However, since ctx->lock is an IRQ
+ * safe lock, and is only held by the CPU doing the modification, having IRQs
+ * disabled is sufficient since it will hold-off the IPIs.
+ */
+#ifdef CONFIG_PROVE_LOCKING
+#define lockdep_assert_event_ctx(event)				\
+	WARN_ON_ONCE(__lockdep_enabled &&			\
+		     (this_cpu_read(hardirqs_enabled) ||	\
+		      lockdep_is_held(&(event)->ctx->mutex) != LOCK_STATE_HELD))
+#else
+#define lockdep_assert_event_ctx(event)
+#endif
+
 #define for_each_sibling_event(sibling, event)			\
+	lockdep_assert_event_ctx(event);			\
 	if ((event)->group_leader == (event))			\
 		list_for_each_entry((sibling), &(event)->sibling_list, sibling_list)
 

kernel/events/core.c

@@ -1468,6 +1468,8 @@ static void __update_context_time(struct perf_event_context *ctx, bool adv)
 {
 	u64 now = perf_clock();
 
+	lockdep_assert_held(&ctx->lock);
+
 	if (adv)
 		ctx->time += now - ctx->timestamp;
 	ctx->timestamp = now;