linux-kernel - Re: [PATCH] dm cache: fix race affecting dirty block count

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Message-ID: <CAJhHMCBEK9bzJaP7tkhTNm8v07dKoxV3FXjNVPFwA2UOHbfn=Q@mail.gmail.com>
Date:	Sun, 3 Aug 2014 01:28:32 -0400
From:	Pranith Kumar <bobby.prani@...il.com>
To:	Anssi Hannula <anssi.hannula@....fi>
Cc:	ejt@...hat.com, snitzer@...hat.com,
	LKML <linux-kernel@...r.kernel.org>,
	torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] dm cache: fix race affecting dirty block count

On Sun, Aug 3, 2014 at 1:17 AM, Pranith Kumar <bobby.prani@...il.com> wrote:
> On 08/03/2014 12:57 AM, Pranith Kumar wrote:
>> On 08/03/2014 12:46 AM, Pranith Kumar wrote:
>>> On 08/02/2014 10:10 PM, Pranith Kumar wrote:
>>>> Corrently adding Anssi this time.
>>>>
>>>> On Sat, Aug 2, 2014 at 10:00 PM, Pranith Kumar <bobby.prani@...il.com> wrote:
>>>>> Hello Anssi, Joe, Mike,
>>>>>
>>>>> I just found your patch in the latest rc and wanted to ask a few
>>>>> questions. I am not sure how this patch helps at all other than luck in
>>>>> that dm_cblock_t type is of type int32_t, which should guarantee that it
>>>>> is atomic on most platforms. Which begs the question, what platform did
>>>>> you encounter this problem?
>>>>>
>>>>> The patch purports to solve a race condition by making use of atomic_t.
>>>>> I am not sure that is enough. If indeed there is a race you need to use
>>>>> smp_mb__{before/after}_atomic() for both your uses of atomic_inc() and
>>>>> atomic_set().
>>>>>
>>>>> Also I have a concern about why this mail was not CC'ed on LKML. I had
>>>>> to go to some difficulty in finding this patch. So please CC LKML for
>>>>> such patches.
>>>>>
>>>>> Thanks,
>>>>> --
>>>>> Pranith
>>>>>
>>>>> -- Begin forwarded Message --
>>>>>
>>>>>
>>>>> nr_dirty is updated without locking, causing it to drift so that it is
>>>>> non-zero (either a small positive integer, or a very large one when an
>>>>> underflow occurs) even when there are no actual dirty blocks.
>>>>>
>>>>> Fix that by using an atomic type for nr_dirty.
>>>>>
>>>>> Signed-off-by: Anssi Hannula <anssi hannula iki fi>
>>>>> Cc: Joe Thornber <ejt redhat com>
>>>>> Cc: stable vger kernel org
>>
>

Bah, please ignore all the previous patches about pointers. It is me
who is drunk and got my pointers mixed up. :(

-- 
Pranith
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