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

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Message-ID: <CAJhHMCC8hDVzeYM1OL1VhTEw=+RrpVK_fQYCbKpnJvBUEP3LyQ@mail.gmail.com>
Date:	Sun, 3 Aug 2014 00:01:17 -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 Sat, Aug 2, 2014 at 11:33 PM, Anssi Hannula <anssi.hannula@....fi> wrote:
> 03.08.2014, 05:10, Pranith Kumar kirjoitti:
>> 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,
>
> Hi,
>
>>> 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?
>
> On x86_64. Regular integer increment/decrement/addition/subtraction are
> not atomic on x86(_64).
>

You are right. Only loads/stores are atomic for an integer. My point
was more about barriers after atomic_inc/dec.

Also dm_cblock_t is uint32_t, but atomic_t changes that to int. You
should correct that to atomic64_t to preserve original semantics.

>>> 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().
>
> The issue was only about concurrent increments/decrements getting lost
> completely from time to time, which atomic_inc/dec will prevent without
> any explicit barriers.
>

These increments and decrements will still be lost if you do not use
barriers in presence of concurrent accesses. Please see
Documentation/memory-barriers.txt.

>>> 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.
>
> I don't usually CC LKML if there is a subsystem-specific mailing list
> unless there is a specific reason to CC LKML as well. I thought this was
> standard practice, but I'm happy to spam LKML as well from now on if
> that is preferred by others as well.
>

I guess it is always a good idea to CC LKML in case of patches in
addition to sub-system mailing lists. For one, it is easy to forward a
message from lkml and then reply to that in case there is something to
discuss.


-- 
Pranith
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