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

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Date:	Mon, 4 Aug 2014 11:48:26 +0100
From:	Joe Thornber <thornber@...hat.com>
To:	Pranith Kumar <bobby.prani@...il.com>
Cc:	Anssi Hannula <anssi.hannula@....fi>, 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 03, 2014 at 12:01:17AM -0400, Pranith Kumar wrote:
> 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.

atomic_t used to have only 24 bits of range due to the Sparc
implementation holding a lock in one of the bytes.  I understand this
limitation was removed during 2.6 and the full 32 bits are now
available.

eg,

    https://github.com/jthornber/linux-2.6/commit/37682177af68478fa83429b735fa16913c2fbb2b

> 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.

You do not need to use barriers for plain atomic_inc/dec().

    https://github.com/jthornber/linux-2.6/blob/thin-dev/Documentation/atomic_ops.txt#L187

You _do_ need to use a memory barrier for the ops that return a value
(such as atomic_dec_and_test()), But only if there's some other state
that needs synchronising.  See the nice example in atomic_ops.txt:

    https://github.com/jthornber/linux-2.6/blob/thin-dev/Documentation/atomic_ops.txt#L321

We just trigger a stateless event when the counter hits zero, so the
patch is fine.

- Joe
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