linux-kernel - Re: [PATCH] make atomic_t volatile on all architectures

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Date:	Thu, 9 Aug 2007 01:25:21 +0200
From:	Lennert Buytenhek <buytenh@...tstofly.org>
To:	Chris Snook <csnook@...hat.com>
Cc:	akpm@...ux-foundation.org, torvalds@...ux-foundation.org,
	ak@...e.de, heiko.carstens@...ibm.com, davem@...emloft.net,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	schwidefsky@...ibm.com, wensong@...ux-vs.org, horms@...ge.net.au,
	wjiang@...ilience.com, cfriesen@...tel.com, zlynx@....org
Subject: Re: [PATCH] make atomic_t volatile on all architectures

On Wed, Aug 08, 2007 at 07:07:33PM -0400, Chris Snook wrote:

> From: Chris Snook <csnook@...hat.com>
> 
> Some architectures currently do not declare the contents of an atomic_t to be
> volatile.  This causes confusion since atomic_read() might not actually read
> anything if an optimizing compiler re-uses a value stored in a register, which
> can break code that loops until something external changes the value of an
> atomic_t.  Avoiding such bugs requires using barrier(), which causes re-loads
> of all registers used in the loop, thus hurting performance instead of helping
> it, particularly on architectures where it's unnecessary.  Since we generally
> want to re-read the contents of an atomic variable on every access anyway,
> let's standardize the behavior across all architectures and avoid the
> performance and correctness problems of requiring the use of barrier() in
> loops that expect atomic_t variables to change externally.  This is relevant
> even on non-smp architectures, since drivers may use atomic operations in
> interrupt handlers.
> 
> Signed-off-by: Chris Snook <csnook@...hat.com>

Documentation/atomic_ops.txt would need updating:

	[...]

	One very important aspect of these two routines is that they DO NOT
	require any explicit memory barriers.  They need only perform the
	atomic_t counter update in an SMP safe manner.
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