Re: [Xen-devel] [PATCH] KVM, XEN: Fix potential race in pvclock code

[Julian Stecklina <jsteckli@xxxxxxxxxxxxxxxxxxxx>]

On 01/16/2014 04:04 PM, Jan Beulich wrote:
> I don't think so - this would only be an issue if the conditions used
> | instead of ||. || implies a sequence point between evaluating the
> left and right sides, and the standard says: "The presence of a
> sequence point between the evaluation of expressions A and B
> implies that every value computation and side effect associated
> with A is sequenced before every value computation and side
> effect associated with B."

This only applies to single-threaded code. Multithreaded code must be
data-race free for that to be true. See

https://lwn.net/Articles/508991/

> And even if there was a problem (i.e. my interpretation of the
> above being incorrect), I don't think you'd need ACCESS_ONCE()
> here: The same local variable can't have two different values in
> two different use sites when there was no intermediate
> assignment to it.

Same comment as above.

Julian

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