io_uring: don't resched with empty task_list

Entering tctx_task_work() with empty task_list is a strange scenario,
that can happen only on rare occasion during task exit, so let's not
check for task_list emptiness in advance and do it do-while style. The
code still correct for the empty case, just would do extra work about
which we don't care.

Do extra step and do the check before cond_resched(), so we don't
resched if have nothing to execute.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/c4173e288e69793d03c7d7ce826f9d28afba718a.1623949695.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -1896,7 +1896,7 @@ static void tctx_task_work(struct callback_head *cb)
 
 	clear_bit(0, &tctx->task_state);
 
-	while (!wq_list_empty(&tctx->task_list)) {
+	while (1) {
 		struct io_wq_work_node *node;
 
 		spin_lock_irq(&tctx->task_lock);
@@ -1917,6 +1917,8 @@ static void tctx_task_work(struct callback_head *cb)
 			req->task_work.func(&req->task_work);
 			node = next;
 		}
+		if (wq_list_empty(&tctx->task_list))
+			break;
 		cond_resched();
 	}