Re: 64-bit atomic ops on 32-bit ARM

[Matthew Fernandez <matthew.fernandez@nicta.com.au>]

Ah, so it's used for operating on 64-bit bitfields. Presumably the meaning of each bit is orthogonal
to the others so the atomicity of the write as a whole is irrelevant. Thanks for the clarification,
Rich.

On 06/02/14 10:06, Rich Felker wrote:
> On Thu, Feb 06, 2014 at 09:56:47AM +1100, Matthew Fernandez wrote:
>> Hi all,
>>
>> The header atomic.h contains a couple of functions for operations on 64-bit types. Despite their
>> name, these do not appear to be atomic to me. For example:
>>
>>   static inline void a_and_64(volatile uint64_t *p, uint64_t v)
>
> The function name is simply a misnomer. It should be called "atomic
> bitclear" (and, for "or", "atomic bitset). That's the only way it's
> actually used. In other words we don't care that the and/or operation
> on both words happen atomically with respect to one another, only that
> each bit individually behaves atomically with respect to being set and
> cleared.
>
> BTW, changing the name and semantics would also allow us to drop from
> performing two atomic operations to just performing on atomic
> operation (on the word that actually needs to change). This would be a
> fairly significant optimization, I think. But for now it's not a bug,
> just a missed optimization.
>
> Rich
>

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