io_uring: remove double poll entry on arm failure

__io_queue_proc() can enqueue both poll entries and still fail
afterwards, so the callers trying to cancel it should also try to remove
the second poll entry (if any).

For example, it may leave the request alive referencing a io_uring
context but not accessible for cancellation:

[  282.599913][ T1620] task:iou-sqp-23145   state:D stack:28720 pid:23155 ppid:  8844 flags:0x00004004
[  282.609927][ T1620] Call Trace:
[  282.613711][ T1620]  __schedule+0x93a/0x26f0
[  282.634647][ T1620]  schedule+0xd3/0x270
[  282.638874][ T1620]  io_uring_cancel_generic+0x54d/0x890
[  282.660346][ T1620]  io_sq_thread+0xaac/0x1250
[  282.696394][ T1620]  ret_from_fork+0x1f/0x30

Cc: [email protected]
Fixes: 18bceab ("io_uring: allow POLL_ADD with double poll_wait() users")
Reported-and-tested-by: [email protected]
Signed-off-by: Pavel Begunkov <[email protected]>
Link: https://lore.kernel.org/r/0ec1228fc5eda4cb524eeda857da8efdc43c331c.1626774457.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <[email protected]>

Author: isilence

fs/io_uring.c

@@ -5113,6 +5113,8 @@ static __poll_t __io_arm_poll_handler(struct io_kiocb *req,
 		ipt->error = -EINVAL;
 
 	spin_lock_irq(&ctx->completion_lock);
+	if (ipt->error)
+		io_poll_remove_double(req);
 	if (likely(poll->head)) {
 		spin_lock(&poll->head->lock);
 		if (unlikely(list_empty(&poll->wait.entry))) {