linux-kernel - Re: [PATCH 1/24] make atomic_read() behave consistently on alpha

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Date:	Thu, 09 Aug 2007 10:53:14 -0400
From:	Chris Snook <csnook@...hat.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	torvalds@...ux-foundation.org, netdev@...r.kernel.org,
	akpm@...ux-foundation.org, ak@...e.de, heiko.carstens@...ibm.com,
	davem@...emloft.net, schwidefsky@...ibm.com, wensong@...ux-vs.org,
	horms@...ge.net.au, wjiang@...ilience.com, cfriesen@...tel.com,
	zlynx@....org, rpjday@...dspring.com, jesper.juhl@...il.com
Subject: Re: [PATCH 1/24] make atomic_read() behave consistently on alpha

Paul E. McKenney wrote:
> Why not the same access-once semantics for atomic_set() as
> for atomic_read()?  As this patch stands, it might introduce
> architecture-specific compiler-induced bugs due to the fact that
> atomic_set() used to imply volatile behavior but no longer does.

When we make the volatile cast in atomic_read(), we're casting an rvalue to 
volatile.  This unambiguously tells the compiler that we want to re-load that 
register from memory.  What's "volatile behavior" for an lvalue?  A write to an 
lvalue already implies an eventual write to memory, so this would be a no-op. 
Maybe you'll write to the register a few times before flushing it to memory, but 
it will happen eventually.  With an rvalue, there's no guarantee that it will 
*ever* load from memory, which is what volatile fixes.

I think what you have in mind is LOCK_PREFIX behavior, which is not the purpose 
of atomic_set.  We use LOCK_PREFIX in the inline assembly for the atomic_* 
operations that read, modify, and write a value, only because it is necessary to 
perform that entire transaction atomically.

	-- Chris
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