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

[Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>]

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.

Regards,
Xiaoguang Wang

>