On Wed, Aug 08, 2007 at 07:07:33PM -0400, Chris Snook wrote:
> From: Chris Snook <[email protected]>
>
> 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. Avoiding such bugs requires using barrier(), which causes re-loads
> of all registers used in the loop, thus hurting performance instead of helping
> it, particularly on architectures where it's unnecessary. Since we generally
> want to re-read the contents of an atomic variable on every access anyway,
> let's standardize the behavior across all architectures and avoid the
> performance and correctness problems of requiring the use of barrier() in
> loops that expect atomic_t variables to change externally. This is relevant
> even on non-smp architectures, since drivers may use atomic operations in
> interrupt handlers.
>
> Signed-off-by: Chris Snook <[email protected]>
Documentation/atomic_ops.txt would need updating:
[...]
One very important aspect of these two routines is that they DO NOT
require any explicit memory barriers. They need only perform the
atomic_t counter update in an SMP safe manner.
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