Re: [PATCH v3 08/13] io_uring: implement fixed buffers registration similar to fixed files

[Pavel Begunkov <asml.silence@gmail.com>]

On 06/01/2021 19:46, Bijan Mottahedeh wrote:
> On 1/4/2021 6:43 PM, Pavel Begunkov wrote:
>> On 18/12/2020 18:07, Bijan Mottahedeh wrote:
>>> Apply fixed_rsrc functionality for fixed buffers support.
>>
>> git generated a pretty messy diff...
> 
> I had tried to break this up a few ways but it didn't work well because I think most of the code changes depend on the io_uring structure changes.  I can look again or if you some idea of how you want to split it, I can do that.

Nah, that's fine, I just may have missed something
without applying it.

> 
>> Because it's do quiesce, fixed read/write access buffers from asynchronous
>> contexts without synchronisation. That won't work anymore, so
>>
>> 1. either we save it in advance, that would require extra req_async
>> allocation for linked fixed rw
>>
>> 2. or synchronise whenever async. But that would mean that a request
>> may get and do IO on two different buffers, that's rotten.
>>
>> 3. do mixed -- lazy, but if do IO then alloc.
>>
>> 3.5 also "synchronise" there would mean uring_lock, that's not welcome,
>> but we can probably do rcu.
> 
> Are you referring to a case where a fixed buffer request can be submitted from async context while those buffers are being unregistered, or something like that?

Yes, io_import_fixed() called from io-wq, and in parallel with
unregister/etc. That may happen in many cases, e.g. linked reqs
or IOSQE_ASYNC.


-- 
Pavel Begunkov