linux-kernel - Re: [External] Re: [PATCH v3 2/3] io_uring: avoid ring quiesce while registering/unregistering eventfd

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Date:   Thu, 3 Feb 2022 22:02:43 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Usama Arif <usama.arif@...edance.com>,
        Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     fam.zheng@...edance.com
Subject: Re: [External] Re: [PATCH v3 2/3] io_uring: avoid ring quiesce while
 registering/unregistering eventfd

On 2/3/22 19:54, Usama Arif wrote:
> On 03/02/2022 19:06, Jens Axboe wrote:
>> On 2/3/22 12:00 PM, Pavel Begunkov wrote:
>>> On 2/3/22 18:29, Jens Axboe wrote:
>>>> On 2/3/22 11:26 AM, Usama Arif wrote:
>>>>> Hmm, maybe i didn't understand you and Pavel correctly. Are you
>>>>> suggesting to do the below diff over patch 3? I dont think that would be
>>>>> correct, as it is possible that just after checking if ctx->io_ev_fd is
>>>>> present unregister can be called by another thread and set ctx->io_ev_fd
>>>>> to NULL that would cause a NULL pointer exception later? In the current
>>>>> patch, the check of whether ev_fd exists happens as the first thing
>>>>> after rcu_read_lock and the rcu_read_lock are extremely cheap i believe.
>>>>
>>>> They are cheap, but they are still noticeable at high requests/sec
>>>> rates. So would be best to avoid them.
>>>>
>>>> And yes it's obviously racy, there's the potential to miss an eventfd
>>>> notification if it races with registering an eventfd descriptor. But
>>>> that's not really a concern, as if you register with inflight IO
>>>> pending, then that always exists just depending on timing. The only
>>>> thing I care about here is that it's always _safe_. Hence something ala
>>>> what you did below is totally fine, as we're re-evaluating under rcu
>>>> protection.
>>>
>>> Indeed, the patch doesn't have any formal guarantees for propagation
>>> to already inflight requests, so this extra unsynchronised check
>>> doesn't change anything.
>>>
>>> I'm still more сurious why we need RCU and extra complexity when
>>> apparently there is no use case for that. If it's only about
>>> initial initialisation, then as I described there is a much
>>> simpler approach.
>>
>> Would be nice if we could get rid of the quiesce code in general, but I
>> haven't done a check to see what'd be missing after this...
>>
> 
> I had checked! I had posted below in in reply to v1 (https://lore.kernel.org/io-uring/02fb0bc3-fc38-b8f0-3067-edd2a525ef29@gmail.com/T/#m5ac7867ac61d86fe62c099be793ffe5a9a334976), but i think it got missed! Copy-pasting here for reference:
> 
> "
> I see that if we remove ring quiesce from the the above 3 opcodes, then
> only IORING_REGISTER_ENABLE_RINGS and IORING_REGISTER_RESTRICTIONS is
> left for ring quiesce. I just had a quick look at those, and from what i
> see we might not need to enter ring quiesce in
> IORING_REGISTER_ENABLE_RINGS as the ring is already disabled at that point?
> And for IORING_REGISTER_RESTRICTIONS if we do a similar approach to

IORING_REGISTER_RESTRICTIONS and IORING_REGISTER_ENABLE_RINGS are simpler,
we can just remove quiesce (i.e. put them into io_register_op_must_quiesce())
without any extra changes.

TL;DR;
That's because IORING_SETUP_R_DISABLED prevents submitting requests
and so there will be no requests until IORING_REGISTER_ENABLE_RINGS is
called. And IORING_REGISTER_RESTRICTIONS works only before
IORING_REGISTER_ENABLE_RINGS was called.


-- 
Pavel Begunkov