Re: [PATCH v3 RESEND] io_uring: add timeout support for io_uring_enter()

[Jens Axboe <axboe@kernel.dk>]

On 11/4/20 2:20 PM, Pavel Begunkov wrote:
> On 04/11/2020 20:28, Jens Axboe wrote:
>> On 11/4/20 1:16 PM, Pavel Begunkov wrote:
>>> On 04/11/2020 19:34, Jens Axboe wrote:
>>>> On 11/4/20 12:27 PM, Pavel Begunkov wrote:
>>>>> On 04/11/2020 18:32, Jens Axboe wrote:
>>>>>> On 11/4/20 10:50 AM, Jens Axboe wrote:
>>>>>>> +struct io_uring_getevents_arg {
>>>>>>> +	sigset_t *sigmask;
>>>>>>> +	struct __kernel_timespec *ts;
>>>>>>> +};
>>>>>>> +
>>>>>>
>>>>>> I missed that this is still not right, I did bring it up in your last
>>>>>> posting though - you can't have pointers as a user API, since the size
>>>>>> of the pointer will vary depending on whether this is a 32-bit or 64-bit
>>>>>> arch (or 32-bit app running on 64-bit kernel).
>>>>>
>>>>> Maybe it would be better 
>>>>>
>>>>> 1) to kill this extra indirection?
>>>>>
>>>>> struct io_uring_getevents_arg {
>>>>> -	sigset_t *sigmask;
>>>>> -	struct __kernel_timespec *ts;
>>>>> +	sigset_t sigmask;
>>>>> +	struct __kernel_timespec ts;
>>>>> };
>>>>>
>>>>> then,
>>>>>
>>>>> sigset_t *sig = (...)arg;
>>>>> __kernel_timespec* ts = (...)(arg + offset);
>>>>
>>>> But then it's kind of hard to know which, if any, of them are set... I
>>>> did think about this, and any solution seemed worse than just having the
>>>> extra indirection.
>>>
>>> struct io_uring_getevents_arg {
>>> 	sigset_t sigmask;
>>> 	u32 mask;
>>> 	struct __kernel_timespec ts;
>>> };
>>>
>>> if size > sizeof(sigmask), then use mask to determine that.
>>> Though, not sure how horrid the rest of the code would be.
>>
>> I'm not saying it's not possible, just that I think the end result would
>> be worse in terms of both kernel code and how the user applications (or
>> liburing) would need to use it. I'd rather sacrifice an extra copy for
>> something that's straight forward (and logical) to use, rather than
>> needing weird setups or hoops to jump through. And this mask vs
>> sizeof(mask) thing seems pretty horrendeous to me :-)
> 
> If you think so, I'll spare my time then :)
> 
>>
>>>> Yeah, not doing the extra indirection would save a copy, but don't think
>>>> it's worth it for this path.
>>>
>>> I much more don't like branching like IORING_ENTER_GETEVENTS_TIMEOUT,
>>> from conceptual point. I may try it out to see how it looks like while
>>> it's still for-next.
>>
>> One thing I think we should change is the name,
>> IORING_ENTER_GETEVENTS_TIMEOUT will quickly be a bad name if we end up
>> adding just one more thing to the struct. Would be better to call it
>> IORING_ENTER_EXTRA_DATA or something, meaning that the sigmask pointer
>> is a pointer to the aux data instead of a sigmask. Better name
>> suggestions welcome...
> 
> _EXT_ARG from extended

Yeah I like that, I'll update it

> Also, a minor one -- s/sigsz/argsz/

Yes, might as well.

-- 
Jens Axboe