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

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Date:	Wed, 08 Aug 2007 19:35:47 -0400
From:	Chris Snook <csnook@...hat.com>
To:	Lennert Buytenhek <buytenh@...tstofly.org>
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

Lennert Buytenhek wrote:
> 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.

Thanks, I was looking for that.  I'll re-send shortly with my comment 
moved there.  People are already using atomic_t in a manner that implies 
the use of memory barriers and interrupt-safety, which is what the patch 
enforces.

	-- Chris
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