Re: [PATCH] io_uring: refacor file register/unregister/update based on sequence

[Jens Axboe <axboe@kernel.dk>]

On 3/23/20 10:45 AM, Xiaoguang Wang wrote:
> hi,
> 
>> On 3/23/20 5:50 AM, Xiaoguang Wang wrote:
>>> While diving into iouring fileset resigster/unregister/update codes,
>>> we found one bug in fileset update codes. Iouring fileset update codes
>>> use a percpu_ref variable to check whether can put previous registered
>>> file, only when the refcnt of the perfcpu_ref variable reachs zero, can
>>> we safely put these files, but this do not work well. If applications
>>> always issue requests continually, this perfcpu_ref will never have an
>>> chance to reach zero, and it'll always be in atomic mode, also will
>>> defeat the gains introduced by fileset register/unresiger/update feature,
>>> which are used to reduce the atomic operation overhead of fput/fget.
>>>
>>> To fix this issue, we remove the percpu_ref related codes, and add two new
>>> counter: sq_seq and cq_seq to struct io_ring_ctx:
>>>      sq_seq: the most recent issued requset sequence number, which is
>>>              protected uring_lock.
>>>      cq_seq: the most recent completed request sequence number, which is
>>>              protected completion_lock.
>>>
>>> When we update fileset(under uring_lock), we record the current sq_seq,
>>> and when cq_seq is greater or equal to recorded sq_seq, we know we can
>>> put previous registered file safely.
>>
>> Maybe I'm misunderstanding the idea here, but what if you have the
>> following:
>>
>> - sq_seq 200, cq_seq 100
>>
>> We have 100 inflight, and an unregister request comes in. I then
>> issue 100 nops, which complete. cq_seq is now 200, but none of the
>> original requests that used the file have completed.
>>
>> What am I missing?
> No, you're right. I had thought requests will be completed in the order
> they are issued, thanks for pointing this.
> As for not using per percpu_ref per registered file, I also worry about
> the memory consume, because the max allowed registered files are 32768.

Yeah, I think we have to be a bit creative here with the solution...
Please continue to think about it, would be great to have a better
solution for this!

-- 
Jens Axboe