Re: io_uring: io_fail_links() should only consider first linked timeout

[Pavel Begunkov <asml.silence@gmail.com>]

On 11/20/2019 1:22 PM, Bob Liu wrote:
> On 11/20/19 4:44 PM, Pavel Begunkov wrote:
>> On 11/20/2019 1:33 AM, Jens Axboe wrote:
>>> We currently clear the linked timeout field if we cancel such a timeout,
>>> but we should only attempt to cancel if it's the first one we see.
>>> Others should simply be freed like other requests, as they haven't
>>> been started yet.
>>>
>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>>
>>> ---
>>>
>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>> index a79ef43367b1..d1085e4e8ae9 100644
>>> --- a/fs/io_uring.c
>>> +++ b/fs/io_uring.c
>>> @@ -937,12 +937,12 @@ static void io_fail_links(struct io_kiocb *req)
>>>  		if ((req->flags & REQ_F_LINK_TIMEOUT) &&
>>>  		    link->submit.sqe->opcode == IORING_OP_LINK_TIMEOUT) {
>>>  			io_link_cancel_timeout(link);
>>> -			req->flags &= ~REQ_F_LINK_TIMEOUT;
>>>  		} else {
>>>  			io_cqring_fill_event(link, -ECANCELED);
>>>  			__io_double_put_req(link);
>>>  		}
>>>  		kfree(sqe_to_free);
>>> +		req->flags &= ~REQ_F_LINK_TIMEOUT;
>>
>> That's not necessary, but maybe would safer to keep. If
>> REQ_F_LINK_TIMEOUT is set, than there was a link timeout request,
>> and for it and only for it io_link_cancel_timeout() will be called.
>>
>> However, this is only true if linked timeout isn't fired. Otherwise,
>> there is another bug, which isn't fixed by either of the patches. We
>> need to clear REQ_F_LINK_TIMEOUT in io_link_timeout_fn() as well.
>>
>> Let: REQ -> L_TIMEOUT1 -> L_TIMEOUT2
>> 1. L_TIMEOUT1 fired before REQ is completed
>>
>> 2. io_link_timeout_fn() removes L_TIMEOUT1 from the list:
>> REQ|REQ_F_LINK_TIMEOUT -> L_TIMEOUT2
>>
>> 3. free_req(REQ) then call io_link_cancel_timeout(L_TIMEOUT2)
>> leaking it (as described in my patch).
>>
>> P.S. haven't tried to test nor reproduce it yet.
>>
> 
> Off topic... I'm reading the code regarding IORING_OP_LINK_TIMEOUT.
> But confused by what's going to happen if userspace submit a request with IORING_OP_LINK_TIMEOUT but not IOSQE_IO_LINK.
> 
It fails in __io_submit_sqe() with -EINVAL. (see default branch in the
switch). As for me, it's better to do it late, as it will generically
handle dependant links (e.g. fail them properly).

-- 
Pavel Begunkov