linux-kernel - Re: [PATCH 2/2]: atomic_t: Remove volatile from atomic

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Message-Id: <20100519.125449.56392211.davem@davemloft.net>
Date:	Wed, 19 May 2010 12:54:49 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	paulmck@...ux.vnet.ibm.com
Cc:	npiggin@...e.de, torvalds@...ux-foundation.org, anton@...ba.org,
	akpm@...ux-foundation.org, willy@...ux.intel.com,
	benh@...nel.crashing.org, paulus@...ba.org,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2]: atomic_t: Remove volatile from atomic_t definition

From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Date: Wed, 19 May 2010 08:01:32 -0700

> On Wed, May 19, 2010 at 11:03:27PM +1000, Nick Piggin wrote:
>> For atomic_read it shouldn't matter unless gcc is *really* bad at it.
>> Ah, for atomic_read, the required semantic is surely ACCESS_ONCE, so
>> that's where the volatile is needed? (maybe it would be clearer to
>> explicitly use ACCESS_ONCE?)
> 
> Explicit use of ACCESS_ONCE() where needed makes a lot of sense to me,
> and allows better code to be generated for initialization and cleanup
> code where no other task has access to the atomic_t.

I agree and I want to see this too, but I think with the tree the size
that it is we have to work backwards at this point.

Existing behavior by default, and optimized cases get tagged by using
a new interface (atomic_read_light(), test_bit{,s}_light(), etc.)
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