Re: io_uring_prep_openat_direct() and link/drain

[Jens Axboe <axboe@kernel.dk>]

On 3/30/22 9:12 AM, Miklos Szeredi wrote:
> On Wed, 30 Mar 2022 at 17:05, Jens Axboe <axboe@kernel.dk> wrote:
>>
>> On 3/30/22 8:58 AM, Miklos Szeredi wrote:
>>> Next issue:  seems like file slot reuse is not working correctly.
>>> Attached program compares reads using io_uring with plain reads of
>>> proc files.
>>>
>>> In the below example it is using two slots alternately but the number
>>> of slots does not seem to matter, read is apparently always using a
>>> stale file (the prior one to the most recent open on that slot).  See
>>> how the sizes of the files lag by two lines:
>>>
>>> root@kvm:~# ./procreads
>>> procreads: /proc/1/stat: ok (313)
>>> procreads: /proc/2/stat: ok (149)
>>> procreads: /proc/3/stat: read size mismatch 313/150
>>> procreads: /proc/4/stat: read size mismatch 149/154
>>> procreads: /proc/5/stat: read size mismatch 150/161
>>> procreads: /proc/6/stat: read size mismatch 154/171
>>> ...
>>>
>>> Any ideas?
>>
>> Didn't look at your code yet, but with the current tree, this is the
>> behavior when a fixed file is used:
>>
>> At prep time, if the slot is valid it is used. If it isn't valid,
>> assignment is deferred until the request is issued.
>>
>> Which granted is a bit weird. It means that if you do:
>>
>> <open fileA into slot 1, slot 1 currently unused><read slot 1>
>>
>> the read will read from fileA. But for:
>>
>> <open fileB into slot 1, slot 1 is fileA currently><read slot 1>
>>
>> since slot 1 is already valid at prep time for the read, the read will
>> be from fileA again.
>>
>> Is this what you are seeing? It's definitely a bit confusing, and the
>> only reason why I didn't change it is because it could potentially break
>> applications. Don't think there's a high risk of that, however, so may
>> indeed be worth it to just bite the bullet and the assignment is
>> consistent (eg always done from the perspective of the previous
>> dependent request having completed).
>>
>> Is this what you are seeing?
> 
> Right, this explains it.   Then the only workaround would be to wait
> for the open to finish before submitting the read, but that would
> defeat the whole point of using io_uring for this purpose.

Honestly, I think we should just change it during this round, making it
consistent with the "slot is unused" use case. The old use case is more
more of a "it happened to work" vs the newer consistent behavior of "we
always assign the file when execution starts on the request".

Let me spin a patch, would be great if you could test.

-- 
Jens Axboe