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To:     Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
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 <e11a0716-eb18-4ce3-9902-3247beafe65a@kernel.dk>
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From:   Pavel Begunkov <asml.silence@gmail.com>
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Subject: Re: [PATCH] io_uring: remove wait loop spurious wakeups
Message-ID: <4004b5bc-edf0-cc8f-8efc-7f848c95f0ba@gmail.com>
Date:   Wed, 9 Oct 2019 01:05:45 +0300
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From: Pavel Begunkov <asml.silence@gmail.com>
To: Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
 linux-kernel@vger.kernel.org
Message-ID: <4004b5bc-edf0-cc8f-8efc-7f848c95f0ba@gmail.com>
Subject: Re: [PATCH] io_uring: remove wait loop spurious wakeups
References: <936cd758d6c694fe1b8b9de050e24cfecdc2e60d.1570489620.git.asml.silence@gmail.com>
 <e11a0716-eb18-4ce3-9902-3247beafe65a@kernel.dk>
 <d035bb1b-e6f0-77db-a434-1761b0a7a142@gmail.com>
 <62a8a6c7-9c5b-c9a4-9c73-c77db87c6637@kernel.dk>
 <99bfb7aa-6980-fc14-32f7-a479dea63eb4@gmail.com>
 <a1f8de23-fcad-7252-cbd4-8f5e617056cd@kernel.dk>
In-Reply-To: <a1f8de23-fcad-7252-cbd4-8f5e617056cd@kernel.dk>

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On 09/10/2019 00:22, Jens Axboe wrote:
> On 10/8/19 2:58 PM, Pavel Begunkov wrote:
>> On 08/10/2019 20:00, Jens Axboe wrote:
>>> On 10/8/19 10:43 AM, Pavel Begunkov wrote:
>>>> On 08/10/2019 06:16, Jens Axboe wrote:
>>>>> On 10/7/19 5:18 PM, Pavel Begunkov (Silence) wrote:
>>>>>> From: Pavel Begunkov <asml.silence@gmail.com>
>>>>>>
>>>>>> Any changes interesting to tasks waiting in io_cqring_wait() are
>>>>>> commited with io_cqring_ev_posted(). However, io_ring_drop_ctx_ref=
s()
>>>>>> also tries to do that but with no reason, that means spurious wake=
ups
>>>>>> every io_free_req() and io_uring_enter().
>>>>>>
>>>>>> Just use percpu_ref_put() instead.
>>>>>
>>>>> Looks good, this is a leftover from when the ctx teardown used
>>>>> the waitqueue as well.
>>>>>
>>>> BTW, is there a reason for ref-counting in struct io_kiocb? I unders=
tand
>>>> the idea behind submission reference, but don't see any actual part
>>>> needing it.
>>>
>>> In short, it's to prevent the completion running before we're done wi=
th
>>> the iocb on the submission side.
>>
>> Yep, that's what I expected. Perhaps I missed something, but what I've=

>> seen following code paths all the way down, it either
>> 1. gets error / completes synchronously and then frees req locally
>> 2. or passes it further (e.g. async list) and never accesses it after
>=20
> As soon as the IO is passed on, it can complete. In fact, it can comple=
te
> even _before_ that call returns. That's the issue. Obviously this isn't=

> true for purely polled IO, but it is true for IRQ based IO.

And the idea was to not use io_kiocb after submission. Except when we kno=
w,
that it won't complete asynchronously (e.g. error), that could be checked=

with return code, I guess.

Anyway, thanks for the explanation!

--=20
Yours sincerely,
Pavel Begunkov


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