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

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Message-Id: <20070809125920.37eb6e6d.wjiang@resilience.com>
Date:	Thu, 9 Aug 2007 12:59:20 +0800
From:	Jerry Jiang <wjiang@...ilience.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Chris Snook <csnook@...hat.com>, 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,
	cfriesen@...tel.com, zlynx@....org
Subject: Re: [PATCH] make atomic_t volatile on all architectures

On Wed, 8 Aug 2007 21:18:25 -0700 (PDT)
Linus Torvalds <torvalds@...ux-foundation.org> 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. 

Why? It's a wart! Is it due to unclear C standard on volatile related point?

Why the *volatile-accesses-in-code* is acceptable, does C standard make it clear?

-- Jerry

> 
> 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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