Re: [PATCH 1/3] io_uring: don't use req->work.creds for inline requests

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

On 26/05/2020 17:59, Xiaoguang Wang wrote:
> hi,
> 
>> On 26/05/2020 09:43, Xiaoguang Wang wrote:
>>> In io_init_req(), if uers requires a new credentials, currently we'll
>>> save it in req->work.creds, but indeed io_wq_work is designed to describe
>>> needed running environment for requests that will go to io-wq, if one
>>> request is going to be submitted inline, we'd better not touch io_wq_work.
>>> Here add a new 'const struct cred *creds' in io_kiocb, if uers requires a
>>> new credentials, inline requests can use it.
>>>
>>> This patch is also a preparation for later patch.
>>
>> What's the difference from keeping only one creds field in io_kiocb (i.e.
>> req->work.creds), but handling it specially (i.e. always initialising)? It will
>> be a lot easier than tossing it around.
>>
>> Also, the patch doubles {get,put}_creds() for sqe->personality case, and that's
>> extra atomics without a good reason.
> You're right, thanks.
> The original motivation for this patch is that it's just a preparation later patch
> "io_uring: avoid whole io_wq_work copy for inline requests", I can use
> io_wq_work.func
> to determine whether to drop io_wq_work in io_req_work_drop_env(), so if
> io_wq_work.func
> is NULL, I don't want io_wq_work has a valid creds.
> I'll look into whether we can just assign req->creds's pointer to
> io_wq_work.creds to
> reduce the atomic operations.

See a comment for the [2/3], can spark some ideas.

It's a bit messy and makes it more difficult to keep in mind -- all that extra
state (i.e. initialised or not) + caring whether func was already set. IMHO, the
nop-test do not really justifies extra complexity, unless the whole stuff is
pretty and clear. Can you benchmark something more realistic? at least
reads/writes to null_blk (completion_nsec=0).

-- 
Pavel Begunkov