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

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Date:	Thu, 09 Aug 2007 03:31:10 -0400
From:	Chris Snook <csnook@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	akpm@...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

Linus Torvalds wrote:
> 
> On Wed, 8 Aug 2007, Chris Snook wrote:
>> 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.
> 
> I'd be *much* happier with "atomic_read()" doing the "volatile" instead.
> 
> The fact is, volatile on data structures is a bug. It's a wart in the C 
> language. It shouldn't be used. 
> 
> Volatile accesses in *code* can be ok, and if we have "atomic_read()" 
> expand to a "*(volatile int *)&(x)->value", then I'd be ok with that.
> 
> But marking data structures volatile just makes the compiler screw up 
> totally, and makes code for initialization sequences etc much worse.
> 
> 		Linus

Fair enough.  Casting to (volatile int *) will give us the behavior people 
expect when using atomic_t without needing to use inefficient barriers.

While we have the hood up, should we convert all the atomic_t's to non-volatile 
and put volatile casts in all the atomic_reads?  I don't know enough about the 
various arches to say with confidence that those changes alone will preserve 
existing behavior.  We might need some arch-specific tweaking of the atomic 
operations.

	-- Chris
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