Re: [PATCH 3/3] io_uring: move *queue_link_head() from common path

From: Jens Axboe <axboe@kernel.dk>
To: Pavel Begunkov <asml.silence@gmail.com>,
	io-uring@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/3] io_uring: move *queue_link_head() from common path
Date: Tue, 17 Dec 2019 11:15:35 -0700	[thread overview]
Message-ID: <812da023-1ebe-fa5c-cd1c-266c9708b881@kernel.dk> (raw)
In-Reply-To: <e7bb74e8-0cbe-40f8-9d10-192a8512e1f7@gmail.com>

On 12/17/19 11:12 AM, Pavel Begunkov wrote:
> On 17/12/2019 21:07, Jens Axboe wrote:
>> On 12/17/19 11:05 AM, Pavel Begunkov wrote:
>>> On 17/12/2019 21:01, Jens Axboe wrote:
>>>> On 12/17/19 10:52 AM, Pavel Begunkov wrote:
>>>>> On 17/12/2019 20:37, Jens Axboe wrote:
>>>>>> On 12/17/19 9:45 AM, Jens Axboe wrote:
>>>>>>> On 12/16/19 4:38 PM, Pavel Begunkov wrote:
>>>>>>>> On 17/12/2019 02:22, Pavel Begunkov wrote:
>>>>>>>>> -	} else if (req->sqe->flags & (IOSQE_IO_LINK|IOSQE_IO_HARDLINK)) {
>>>>>>>>> +
>>>>>>>>> +		/* last request of a link, enqueue the link */
>>>>>>>>> +		if (!(sqe_flags & IOSQE_IO_LINK)) {
>>>>>>>>
>>>>>>>> This looks suspicious (as well as in the current revision). Returning back
>>>>>>>> to my questions a few days ago can sqe->flags have IOSQE_IO_HARDLINK, but not
>>>>>>>> IOSQE_IO_LINK? I don't find any check.
>>>>>>>>
>>>>>>>> In other words, should it be as follows?
>>>>>>>> !(sqe_flags & (IOSQE_IO_LINK|IOSQE_IO_HARDLINK))
>>>>>>>
>>>>>>> Yeah, I think that should check for both. I'm fine with either approach
>>>>>>> in general:
>>>>>>>
>>>>>>> - IOSQE_IO_HARDLINK must have IOSQE_IO_LINK set
>>>>>>>
>>>>>>> or
>>>>>>>
>>>>>>> - IOSQE_IO_HARDLINK implies IOSQE_IO_LINK
>>>>>>>
>>>>>>> Seems like the former is easier to verify in terms of functionality,
>>>>>>> since we can rest easy if we check this early and -EINVAL if that isn't
>>>>>>> the case.
>>>>>>>
>>>>>>> What do you think?
>>>>>>
>>>>>> If you agree, want to send in a patch for that for 5.5? Then I can respin
>>>>>> for-5.6/io_uring on top of that, and we can apply your cleanups there.
>>>>>>
>>>>> Yes, that's the idea. Already got a patch, if you haven't done it yet.
>>>>
>>>> I haven't.
>>>>
>>>>> Just was thinking, whether to add a check for not setting both flags
>>>>> at the same moment in the "imply" case. Would give us 1 state in 2 bits
>>>>> for future use.
>>>>
>>>> Not sure I follow what you're saying here, can you elaborate?
>>>>
>>>
>>> Sure
>>>
>>> #define IOSQE_IO_LINK		(1U << 2)	/* links next sqe */
>>> #define IOSQE_IO_HARDLINK	(1U << 3)	/* like LINK, but stronger */
>>>
>>> That's 2 consequent bits, so 4 states:
>>> 0,0 -> not a link
>>> 1,0 -> common link
>>> 0,1 -> hard link
>>> 1,1 -> reserved, space for another link-quirk type
>>>
>>> But that would require additional check, i.e.
>>>
>>> if (flags&(LINK|HARDLINK) == (LINK|HARDLINK)) ...
>>
>> Ah, I see. In terms of usability, I think it makes more sense to have
>>
>> IOSQE_LINK | IOSQE_HARDLINK
>>
>> be the same as just IOSQE_LINK. It would be nice to retain that for
> 
> Probably, you meant it to be the same as __IOSQE_HARDLINK__
> 
>> something else, but I think it'll be more confusing to users.
>>
> 
> Yeah, and it's easier for something like:
> 
> sqe->flags |= IOSQE_LINK;
> [some code]
> if (timer_or_whatever())
> 	sqe->flags |= IOSQE_HARDLINK;

Precisely. So let's keep it as-is.

-- 
Jens Axboe