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To:     Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
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From:   Pavel Begunkov <asml.silence@gmail.com>
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Subject: Re: [BUG] io_uring: defer logic based on shared data
Message-ID: <31a7765b-bb6d-985a-454d-d998678100d1@gmail.com>
Date:   Fri, 25 Oct 2019 19:40:29 +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: <31a7765b-bb6d-985a-454d-d998678100d1@gmail.com>
Subject: Re: [BUG] io_uring: defer logic based on shared data
References: <5badf1c0-9a7d-0950-2943-ff8db33e0929@gmail.com>
 <bfb58429-6abe-06f0-3fd8-14a0040cecf0@kernel.dk>
 <b44b0488-ba66-0187-2d9b-6949ceb613fb@gmail.com>
 <96446fe1-4f32-642b-7100-ebfa291d7127@kernel.dk>
 <df3b9edd-86ad-5460-b61b-66707c0fb630@kernel.dk>
In-Reply-To: <df3b9edd-86ad-5460-b61b-66707c0fb630@kernel.dk>

--DvpRx4KljX6yzYynEe9WpMbne6GOEA6TB
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On 25/10/2019 19:32, Jens Axboe wrote:
> On 10/25/19 10:27 AM, Jens Axboe wrote:
>> On 10/25/19 10:21 AM, Pavel Begunkov wrote:
>>> On 25/10/2019 19:03, Jens Axboe wrote:
>>>> On 10/25/19 3:55 AM, Pavel Begunkov wrote:
>>>>> I found 2 problems with __io_sequence_defer().
>>>>>
>>>>> 1. it uses @sq_dropped, but doesn't consider @cq_overflow
>>>>> 2. @sq_dropped and @cq_overflow are write-shared with userspace, so=

>>>>> it can be maliciously changed.
>>>>>
>>>>> see sent liburing test (test/defer *_hung()), which left an unkilla=
ble
>>>>> process for me
>>>>
>>>> OK, how about the below. I'll split this in two, as it's really two
>>>> separate fixes.
>>> cached_sq_dropped is good, but I was concerned about cached_cq_overfl=
ow.
>>> io_cqring_fill_event() can be called in async, so shouldn't we do som=
e
>>> synchronisation then?
>>
>> We should probably make it an atomic just to be on the safe side, I'll=

>> update the series.
>=20
> Here we go, patch 1:
>=20
> http://git.kernel.dk/cgit/linux-block/commit/?h=3Dfor-linus&id=3Df2a241=
f596ed9e12b7c8f960e79ccda8053ea294
>=20
> patch 2:
>=20
> http://git.kernel.dk/cgit/linux-block/commit/?h=3Dfor-linus&id=3Db7d029=
7d2df5bfa0d1ecf9d6c66d23676751ef6a
>=20
1. submit rqs (not yet completed)
2. poll_list is empty, inflight =3D 0
3. async completed and placed into poll_list

So, poll_list is not empty, but we won't get to polling again.
At least until someone submitted something.

--=20
Yours sincerely,
Pavel Begunkov


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