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

[Jens Axboe <axboe@kernel.dk>]

On 11/20/19 7:03 AM, Bob Liu wrote:
> On 11/20/19 7:07 PM, Pavel Begunkov wrote:
>> 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).
>>
> 
> I see, thanks.
> As for me, it may better return -EINVAL in advance so as to skip a lot
> unnecessary code for those reqs.

It's an error case, hence unnecessary code isn't an issue. It's much
more important to just let it unwind naturally for that, imho.

-- 
Jens Axboe