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

From: Chris Snook
Date: Thu Aug 09 2007 - 10:54:37 EST


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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