io_uring: prolong tctx_task_work() with flushing

io_submit_flush_completions() may enqueue linked requests for task_work
execution, so don't leave tctx_task_work() right after the tw list is
exhausted, but try to flush and then retry.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/0755d4c2c36301447c63bdd4146c10477cea4249.1630539342.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -2102,6 +2102,9 @@ static void tctx_task_work(struct callback_head *cb)
 	while (1) {
 		struct io_wq_work_node *node;
 
+		if (!tctx->task_list.first && locked && ctx->submit_state.compl_nr)
+			io_submit_flush_completions(ctx);
+
 		spin_lock_irq(&tctx->task_lock);
 		node = tctx->task_list.first;
 		INIT_WQ_LIST(&tctx->task_list);