Re: [Xen-devel] mixture of atomic and non-atomic operations on CPU masks

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 10/09/14 07:46, Jan Beulich wrote:
> All,
>
> the inconsistency here has been puzzling me for a while, and I think
> we ought to do something about it (perhaps not for 4.5 anymore,
> but then right after): cpumask_(test_and_)?(set|clear)_cpu all use
> the respective atomic bitops, in contrast to all other operations on
> cpumask_t. A good part of the users don't require the atomicity at
> all (in particular any of those acting on function scope variables).
> Does anyone know of reasons why the default shouldn't be non-
> atomic ops across the board, with atomic special cases being made
> available for the few cases where they're actually needed?

Its entirely possible that the first users of test_and_? required
atomicity, and the first users which didn't require atomicity copied the
atomic ops as they already existed.

Defaulting to non-atomic seems reasonable, as I suspect that there are
fairly few cases where atomicity is required.  (At least atomic
operations on the local stack is not especially bad to other cpus; I
really hope other cpus don't have writable cache lines of our stack.)

~Andrew


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