[PATCH v3 3/7] perf: Simplify perf_event_free_task() wait

[Peter Zijlstra <peterz@infradead.org>]

Simplify the code by moving the duplicated wakeup condition into
put_ctx().

Notably, wait_var_event() is in perf_event_free_task() and will have
set ctx->task = TASK_TOMBSTONE.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/events/core.c |   32 ++++++++------------------------
 1 file changed, 8 insertions(+), 24 deletions(-)

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -1223,8 +1223,14 @@ static void put_ctx(struct perf_event_co
 	if (refcount_dec_and_test(&ctx->refcount)) {
 		if (ctx->parent_ctx)
 			put_ctx(ctx->parent_ctx);
-		if (ctx->task && ctx->task != TASK_TOMBSTONE)
-			put_task_struct(ctx->task);
+		if (ctx->task) {
+			if (ctx->task == TASK_TOMBSTONE) {
+				smp_mb(); /* pairs with wait_var_event() */
+				wake_up_var(&ctx->refcount);
+			} else {
+				put_task_struct(ctx->task);
+			}
+		}
 		call_rcu(&ctx->rcu_head, free_ctx);
 	}
 }
@@ -5492,8 +5498,6 @@ int perf_event_release_kernel(struct per
 again:
 	mutex_lock(&event->child_mutex);
 	list_for_each_entry(child, &event->child_list, child_list) {
-		void *var = NULL;
-
 		/*
 		 * Cannot change, child events are not migrated, see the
 		 * comment with perf_event_ctx_lock_nested().
@@ -5533,39 +5537,19 @@ int perf_event_release_kernel(struct per
 			 * this can't be the last reference.
 			 */
 			put_event(event);
-		} else {
-			var = &ctx->refcount;
 		}
 
 		mutex_unlock(&event->child_mutex);
 		mutex_unlock(&ctx->mutex);
 		put_ctx(ctx);
 
-		if (var) {
-			/*
-			 * If perf_event_free_task() has deleted all events from the
-			 * ctx while the child_mutex got released above, make sure to
-			 * notify about the preceding put_ctx().
-			 */
-			smp_mb(); /* pairs with wait_var_event() */
-			wake_up_var(var);
-		}
 		goto again;
 	}
 	mutex_unlock(&event->child_mutex);
 
 	list_for_each_entry_safe(child, tmp, &free_list, child_list) {
-		void *var = &child->ctx->refcount;
-
 		list_del(&child->child_list);
 		free_event(child);
-
-		/*
-		 * Wake any perf_event_free_task() waiting for this event to be
-		 * freed.
-		 */
-		smp_mb(); /* pairs with wait_var_event() */
-		wake_up_var(var);
 	}
 
 no_ctx: