Re: [PATCH v4 2/6] perf: Enqueue SIGTRAP always via task_work.

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2024-10-30 16:46:22 [+0100], Frederic Weisbecker wrote:
> This needs more thoughts. We must make sure that the parent is put _after_
> the child because it's dereferenced on release, for example:
…
> put_event()
>    free_event()
>       irq_work_sync(&event->pending_irq);
>       ====> IRQ or irq_workd
>       perf_event_wakeup()
>          ring_buffer_wakeup()
> 	    event = event->parent;
> 	    rcu_dereference(event->rb);
> 
> And now after this patch it's possible that this happens after
> the parent has been released.
> 
> We could put the parent from the child's free_event() but some
> places (inherit_event()) may call free_event() on a child without
> having held a reference to the parent.
> 
> Also note that with this patch the task may receive late irrelevant
> signals after the event is removed. It's probably not that bad but
> still... This could be a concern for exec(), is there a missing
> task_work_run() there before flush_signal_handlers()?

So if this causes so much pain, what about taking only one item at a
item? The following passes the test, too:

diff --git a/kernel/task_work.c b/kernel/task_work.c
index c969f1f26be58..fc796ffddfc74 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -206,7 +206,7 @@ bool task_work_cancel(struct task_struct *task, struct callback_head *cb)
 void task_work_run(void)
 {
 	struct task_struct *task = current;
-	struct callback_head *work, *head, *next;
+	struct callback_head *work, *head;
 
 	for (;;) {
 		/*
@@ -214,17 +214,7 @@ void task_work_run(void)
 		 * work_exited unless the list is empty.
 		 */
 		work = READ_ONCE(task->task_works);
-		do {
-			head = NULL;
-			if (!work) {
-				if (task->flags & PF_EXITING)
-					head = &work_exited;
-				else
-					break;
-			}
-		} while (!try_cmpxchg(&task->task_works, &work, head));
-
-		if (!work)
+		if (!work && !(task->flags & PF_EXITING))
 			break;
 		/*
 		 * Synchronize with task_work_cancel_match(). It can not remove
@@ -232,13 +222,24 @@ void task_work_run(void)
 		 * But it can remove another entry from the ->next list.
 		 */
 		raw_spin_lock_irq(&task->pi_lock);
+		do {
+			head = NULL;
+			if (work) {
+				head = READ_ONCE(work->next);
+			} else {
+				if (task->flags & PF_EXITING)
+					head = &work_exited;
+				else
+					break;
+			}
+		} while (!try_cmpxchg(&task->task_works, &work, head));
 		raw_spin_unlock_irq(&task->pi_lock);
 
-		do {
-			next = work->next;
-			work->func(work);
-			work = next;
+		if (!work)
+			break;
+		work->func(work);
+
+		if (head)
 			cond_resched();
-		} while (work);
 	}
 }

Sebastian