io_uring: retry poll if we got woken with non-matching mask

If we get woken and the poll doesn't match our mask, re-add the task
to the poll waitqueue and try again instead of completing the request
with a mask of 0.
Reported-by: Dan Melnic <dmm@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -4412,8 +4412,20 @@ static void io_poll_complete(struct io_kiocb *req, __poll_t mask, int error)
 static void io_poll_task_handler(struct io_kiocb *req, struct io_kiocb **nxt)
 {
 	struct io_ring_ctx *ctx = req->ctx;
+	struct io_poll_iocb *poll = &req->poll;
+
+	if (!req->result && !READ_ONCE(poll->canceled)) {
+		struct poll_table_struct pt = { ._key = poll->events };
+
+		req->result = vfs_poll(req->file, &pt) & poll->events;
+	}
 
 	spin_lock_irq(&ctx->completion_lock);
+	if (!req->result && !READ_ONCE(poll->canceled)) {
+		add_wait_queue(poll->head, &poll->wait);
+		spin_unlock_irq(&ctx->completion_lock);
+		return;
+	}
 	hash_del(&req->hash_node);
 	io_poll_complete(req, req->result, 0);
 	req->flags |= REQ_F_COMP_LOCKED;