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

From: Chris Snook <[email-masked]>
Date: Thu, 09 Aug 2007 10:53:14 -0400

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