io_uring/poll: fix double poll req->flags races

io_poll_double_prepare()            | io_poll_wake()
                                    | poll->head = NULL
smp_load(&poll->head); /* NULL */   |
flags = req->flags;                 |
                                    | req->flags &= ~SINGLE_POLL;
req->flags = flags | DOUBLE_POLL    |

The idea behind io_poll_double_prepare() is to serialise with the
first poll entry by taking the wq lock. However, it's not safe to assume
that io_poll_wake() is not running when we can't grab the lock and so we
may race modifying req->flags.

Skip double poll setup if that happens. It's ok because the first poll
entry will only be removed when it's definitely completing, e.g.
pollfree or oneshot with a valid mask.

Fixes: 49f1c68e ("io_uring: optimise submission side poll_refs")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/b7fab2d502f6121a7d7b199fe4d914a43ca9cdfd.1668184658.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/poll.c

@@ -394,7 +394,8 @@ static int io_poll_wake(struct wait_queue_entry *wait, unsigned mode, int sync,
 	return 1;
 }
 
-static void io_poll_double_prepare(struct io_kiocb *req)
+/* fails only when polling is already completing by the first entry */
+static bool io_poll_double_prepare(struct io_kiocb *req)
 {
 	struct wait_queue_head *head;
 	struct io_poll *poll = io_poll_get_single(req);
@@ -403,20 +404,20 @@ static void io_poll_double_prepare(struct io_kiocb *req)
 	rcu_read_lock();
 	head = smp_load_acquire(&poll->head);
 	/*
-	 * poll arm may not hold ownership and so race with
-	 * io_poll_wake() by modifying req->flags. There is only one
-	 * poll entry queued, serialise with it by taking its head lock.
+	 * poll arm might not hold ownership and so race for req->flags with
+	 * io_poll_wake(). There is only one poll entry queued, serialise with
+	 * it by taking its head lock. As we're still arming the tw hanlder
+	 * is not going to be run, so there are no races with it.
 	 */
-	if (head)
+	if (head) {
 		spin_lock_irq(&head->lock);
-
 		req->flags |= REQ_F_DOUBLE_POLL;
 		if (req->opcode == IORING_OP_POLL_ADD)
 			req->flags |= REQ_F_ASYNC_DATA;
-
-	if (head)
 		spin_unlock_irq(&head->lock);
+	}
 	rcu_read_unlock();
+	return !!head;
 }
 
 static void __io_queue_proc(struct io_poll *poll, struct io_poll_table *pt,
@@ -454,7 +455,11 @@ static void __io_queue_proc(struct io_poll *poll, struct io_poll_table *pt,
 		/* mark as double wq entry */
 		wqe_private |= IO_WQE_F_DOUBLE;
 		io_init_poll_iocb(poll, first->events, first->wait.func);
-		io_poll_double_prepare(req);
+		if (!io_poll_double_prepare(req)) {
+			/* the request is completing, just back off */
+			kfree(poll);
+			return;
+		}
 		*poll_ptr = poll;
 	} else {
 		/* fine to modify, there is no poll queued to race with us */