io_uring: enable optimized link handling for IORING_OP_POLL_ADD

As introduced by commit:

ba816ad6 ("io_uring: run dependent links inline if possible")

enable inline dependent link running for poll commands.
io_poll_complete_work() is the most important change, as it allows a
linked sequence of { POLL, READ } (for example) to proceed inline
instead of needing to get punted to another async context. The
submission side only potentially matters for sqthread, but may as well
include that bit.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -1806,6 +1806,7 @@ static void io_poll_complete_work(struct io_wq_work **workptr)
 	struct io_poll_iocb *poll = &req->poll;
 	struct poll_table_struct pt = { ._key = poll->events };
 	struct io_ring_ctx *ctx = req->ctx;
+	struct io_kiocb *nxt = NULL;
 	__poll_t mask = 0;
 
 	if (work->flags & IO_WQ_WORK_CANCEL)
@@ -1832,7 +1833,10 @@ static void io_poll_complete_work(struct io_wq_work **workptr)
 	spin_unlock_irq(&ctx->completion_lock);
 
 	io_cqring_ev_posted(ctx);
-	io_put_req(req, NULL);
+
+	io_put_req(req, &nxt);
+	if (nxt)
+		*workptr = &nxt->work;
 }
 
 static int io_poll_wake(struct wait_queue_entry *wait, unsigned mode, int sync,
@@ -1886,7 +1890,8 @@ static void io_poll_queue_proc(struct file *file, struct wait_queue_head *head,
 	add_wait_queue(head, &pt->req->poll.wait);
 }
 
-static int io_poll_add(struct io_kiocb *req, const struct io_uring_sqe *sqe)
+static int io_poll_add(struct io_kiocb *req, const struct io_uring_sqe *sqe,
+		       struct io_kiocb **nxt)
 {
 	struct io_poll_iocb *poll = &req->poll;
 	struct io_ring_ctx *ctx = req->ctx;
@@ -1949,7 +1954,7 @@ static int io_poll_add(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 
 	if (mask) {
 		io_cqring_ev_posted(ctx);
-		io_put_req(req, NULL);
+		io_put_req(req, nxt);
 	}
 	return ipt.error;
 }
@@ -2238,7 +2243,7 @@ static int __io_submit_sqe(struct io_ring_ctx *ctx, struct io_kiocb *req,
 		ret = io_fsync(req, s->sqe, nxt, force_nonblock);
 		break;
 	case IORING_OP_POLL_ADD:
-		ret = io_poll_add(req, s->sqe);
+		ret = io_poll_add(req, s->sqe, nxt);
 		break;
 	case IORING_OP_POLL_REMOVE:
 		ret = io_poll_remove(req, s->sqe);