io_uring: refactor tctx task_work list splicing

We don't need a full copy of tctx->task_list in tctx_task_work(), but
only a first one, so just assign node directly.

Taking into account that task_works are run in a context of a task,
it's very unlikely to first see non-empty tctx->task_list and then
splice it empty, can only happen with task_work cancellations that is
not-normal slow path anyway. Hence, get rid of the check in the end,
it's there not for validity but "performance" purposes.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/d076c83fedb8253baf43acb23b8fafd7c5da1714.1623949695.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -1897,15 +1897,13 @@ static void tctx_task_work(struct callback_head *cb)
 	clear_bit(0, &tctx->task_state);
 
 	while (!wq_list_empty(&tctx->task_list)) {
-		struct io_wq_work_list list;
 		struct io_wq_work_node *node;
 
 		spin_lock_irq(&tctx->task_lock);
-		list = tctx->task_list;
+		node = tctx->task_list.first;
 		INIT_WQ_LIST(&tctx->task_list);
 		spin_unlock_irq(&tctx->task_lock);
 
-		node = list.first;
 		while (node) {
 			struct io_wq_work_node *next = node->next;
 			struct io_kiocb *req = container_of(node, struct io_kiocb,
@@ -1919,9 +1917,6 @@ static void tctx_task_work(struct callback_head *cb)
 			req->task_work.func(&req->task_work);
 			node = next;
 		}
-
-		if (!list.first)
-			break;
 		cond_resched();
 	}