Re: [RFC][PATCH v2 5/5] sched: User Mode Concurency Groups

On Thu, Jan 20, 2022 at 04:55:22PM +0100, Peter Zijlstra wrote:

+SYSCALL_DEFINE2(umcg_wait, u32, flags, u64, timo)
+{
+	struct task_struct *tsk = current;
+	struct umcg_task __user *self = READ_ONCE(tsk->umcg_task);
+	bool worker = tsk->flags & PF_UMCG_WORKER;
+	int ret;
+
+	if (!self || flags)
+		return -EINVAL;
+
+	if (worker) {
+		tsk->flags &= ~PF_UMCG_WORKER;
+		if (timo)
+			return -ERANGE;
+	}
+
+	/* see umcg_sys_{enter,exit}() syscall exceptions */
+	ret = umcg_pin_pages();
+	if (ret)
+		goto unblock;
+
+	/*
+	 * Clear UMCG_TF_COND_WAIT *and* check state == RUNNABLE.
+	 */
+	ret = umcg_update_state(tsk, self, UMCG_TASK_RUNNABLE, UMCG_TASK_RUNNABLE);
+	if (ret)
+		goto unpin;
+
+	ret = umcg_wake_next(tsk, self);
+	if (ret)
+		goto unpin;
+
+	if (worker) {
+		/*
+		 * If this fails it is possible ::next_tid is already running
+		 * while this task is not going to block. This violates our
+		 * constraints.
+		 *
+		 * That said, pretty much the only way to make this fail is by
+		 * force munmap()'ing things. In which case one is most welcome
+		 * to the pieces.
+		 */
+		ret = umcg_enqueue_and_wake(tsk);
+		if (ret)
+			goto unpin;
+	}
+
+	umcg_unpin_pages();
+
+	ret = umcg_wait(timo);
+	switch (ret) {
+	case 0:		/* all done */
+	case -EINTR:	/* umcg_notify_resume() will continue the wait */

So I was playing with the whole worker timeout thing last night and
realized this is broken. If we get a signal while we have a timeout, the
timeout gets lost.

I think the easiest solution is to have umcg_notify_resume() also resume
the timeout, but the first pass of that was yuck, so I need to try
again.

Related, by moving the whole enqueue-and-wake thing into the timeout, we
get more 'fun' failure cases :-(

Oh well..

+		ret = 0;
+		break;
+
+	default:
+		goto unblock;
+	}
+out:
+	if (worker)
+		tsk->flags |= PF_UMCG_WORKER;
+	return ret;
+
+unpin:
+	umcg_unpin_pages();
+unblock:
+	umcg_update_state(tsk, self, UMCG_TASK_RUNNABLE, UMCG_TASK_RUNNING);
+	goto out;
+}