RE: [PATCH v2 2/6] bitops: always define asm-generic non-atomic bitops

David Laight
2022-06-13T21:31:27Z

From: Luck, Tony
> Sent: 13 June 2022 17:27
>
> >> It's listed in Documentation/atomic_bitops.txt.
> >
> > Oh, so my memory was actually correct that I saw it in the docs
> > somewhere.
> > WDYT, should I mention this here in the code (block comment) as well
> > that it's atomic and must not lose `volatile` as Andy suggested or
> > it's sufficient to have it in the docs (+ it's not underscored)?
>
> I think a comment that the "volatile" is required to prevent re-ordering
> is enough.
>
> But maybe others are sufficiently clear on the meaning? I once wasted
> time looking for the non-atomic __test_bit() version (to use in some code
> that was already protected by a spin lock, so didn't need the overhead
> of an "atomic" version) before realizing there wasn't a non-atomic one.

Does it make any sense for 'test bit' to be atomic?

I'm not even sure is needs any ordering constraints either.
The result is always stale - the value can be changed by
another cpu at any time.

The set/clear atomic bit-ops require a RMW bus cycle - which has
to be locked (or similar) to avoid corruption.

The atomic 'test and set' (etc) are RMW and return a valid state.

David

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