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

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Message-ID: <alpine.LFD.0.999.0708082116060.25146@woody.linux-foundation.org>
Date:	Wed, 8 Aug 2007 21:18:25 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Chris Snook <csnook@...hat.com>
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



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
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