perf: Rename perf_event_exit_task(.child)

The task passed to perf_event_exit_task() is not a child, it is
current. Fix this confusing naming, since much of the rest of the code
also relies on it being current.

Specifically, both exec() and exit() callers use it with current as
the argument.

Notably, task_ctx_sched_out() doesn't make much sense outside of
current.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Ravi Bangoria <ravi.bangoria@amd.com>
Link: https://lkml.kernel.org/r/20250307193305.486326750@infradead.org

Author: Peter Zijlstra

kernel/events/core.c

@@ -13742,13 +13742,13 @@ perf_event_exit_event(struct perf_event *event, struct perf_event_context *ctx)
 	perf_event_wakeup(event);
 }
 
-static void perf_event_exit_task_context(struct task_struct *child, bool exit)
+static void perf_event_exit_task_context(struct task_struct *task, bool exit)
 {
-	struct perf_event_context *child_ctx, *clone_ctx = NULL;
+	struct perf_event_context *ctx, *clone_ctx = NULL;
 	struct perf_event *child_event, *next;
 
-	child_ctx = perf_pin_task_context(child);
-	if (!child_ctx)
+	ctx = perf_pin_task_context(task);
+	if (!ctx)
 		return;
 
 	/*
@@ -13761,28 +13761,28 @@ static void perf_event_exit_task_context(struct task_struct *child, bool exit)
 	 * without ctx::mutex (it cannot because of the move_group double mutex
 	 * lock thing). See the comments in perf_install_in_context().
 	 */
-	mutex_lock(&child_ctx->mutex);
+	mutex_lock(&ctx->mutex);
 
 	/*
 	 * In a single ctx::lock section, de-schedule the events and detach the
 	 * context from the task such that we cannot ever get it scheduled back
 	 * in.
 	 */
-	raw_spin_lock_irq(&child_ctx->lock);
+	raw_spin_lock_irq(&ctx->lock);
 	if (exit)
-		task_ctx_sched_out(child_ctx, NULL, EVENT_ALL);
+		task_ctx_sched_out(ctx, NULL, EVENT_ALL);
 
 	/*
 	 * Now that the context is inactive, destroy the task <-> ctx relation
 	 * and mark the context dead.
 	 */
-	RCU_INIT_POINTER(child->perf_event_ctxp, NULL);
-	put_ctx(child_ctx); /* cannot be last */
-	WRITE_ONCE(child_ctx->task, TASK_TOMBSTONE);
-	put_task_struct(child); /* cannot be last */
+	RCU_INIT_POINTER(task->perf_event_ctxp, NULL);
+	put_ctx(ctx); /* cannot be last */
+	WRITE_ONCE(ctx->task, TASK_TOMBSTONE);
+	put_task_struct(task); /* cannot be last */
 
-	clone_ctx = unclone_ctx(child_ctx);
-	raw_spin_unlock_irq(&child_ctx->lock);
+	clone_ctx = unclone_ctx(ctx);
+	raw_spin_unlock_irq(&ctx->lock);
 
 	if (clone_ctx)
 		put_ctx(clone_ctx);
@@ -13793,12 +13793,12 @@ static void perf_event_exit_task_context(struct task_struct *child, bool exit)
 	 * get a few PERF_RECORD_READ events.
 	 */
 	if (exit)
-		perf_event_task(child, child_ctx, 0);
+		perf_event_task(task, ctx, 0);
 
-	list_for_each_entry_safe(child_event, next, &child_ctx->event_list, event_entry)
-		perf_event_exit_event(child_event, child_ctx);
+	list_for_each_entry_safe(child_event, next, &ctx->event_list, event_entry)
+		perf_event_exit_event(child_event, ctx);
 
-	mutex_unlock(&child_ctx->mutex);
+	mutex_unlock(&ctx->mutex);
 
 	if (!exit) {
 		/*
@@ -13814,24 +13814,26 @@ static void perf_event_exit_task_context(struct task_struct *child, bool exit)
 		 *
 		 * Wait for all events to drop their context reference.
 		 */
-		wait_var_event(&child_ctx->refcount,
-			       refcount_read(&child_ctx->refcount) == 1);
+		wait_var_event(&ctx->refcount,
+			       refcount_read(&ctx->refcount) == 1);
 	}
-	put_ctx(child_ctx);
+	put_ctx(ctx);
 }
 
 /*
- * When a child task exits, feed back event values to parent events.
+ * When a task exits, feed back event values to parent events.
  *
  * Can be called with exec_update_lock held when called from
  * setup_new_exec().
  */
-void perf_event_exit_task(struct task_struct *child)
+void perf_event_exit_task(struct task_struct *task)
 {
 	struct perf_event *event, *tmp;
 
-	mutex_lock(&child->perf_event_mutex);
-	list_for_each_entry_safe(event, tmp, &child->perf_event_list,
+	WARN_ON_ONCE(task != current);
+
+	mutex_lock(&task->perf_event_mutex);
+	list_for_each_entry_safe(event, tmp, &task->perf_event_list,
 				 owner_entry) {
 		list_del_init(&event->owner_entry);
 
@@ -13842,23 +13844,23 @@ void perf_event_exit_task(struct task_struct *child)
 		 */
 		smp_store_release(&event->owner, NULL);
 	}
-	mutex_unlock(&child->perf_event_mutex);
+	mutex_unlock(&task->perf_event_mutex);
 
-	perf_event_exit_task_context(child, true);
+	perf_event_exit_task_context(task, true);
 
 	/*
 	 * The perf_event_exit_task_context calls perf_event_task
-	 * with child's task_ctx, which generates EXIT events for
-	 * child contexts and sets child->perf_event_ctxp[] to NULL.
+	 * with task's task_ctx, which generates EXIT events for
+	 * task contexts and sets task->perf_event_ctxp[] to NULL.
 	 * At this point we need to send EXIT events to cpu contexts.
 	 */
-	perf_event_task(child, NULL, 0);
+	perf_event_task(task, NULL, 0);
 
 	/*
 	 * Detach the perf_ctx_data for the system-wide event.
 	 */
 	guard(percpu_read)(&global_ctx_data_rwsem);
-	detach_task_ctx_data(child);
+	detach_task_ctx_data(task);
 }
 
 /*