Re: [Xen-devel] [PATCH 00/10] [PATCH RFC V2] Paravirtualized ticketlocks

[Jeremy Fitzhardinge]

On 09/28/2011 11:49 AM, Linus Torvalds wrote:
> But I don't care all *that* deeply. I do agree that the xaddw trick is
> pretty tricky. I just happen to think that it's actually *less* tricky
> than "read the upper bits separately and depend on subtle ordering
> issues with another writer that happens at the same time on another
> CPU".
>
> So I can live with either form - as long as it works. I think it might
> be easier to argue that the xaddw is guaranteed to work, because all
> values at all points are unarguably atomic (yeah, we read the lower
> bits nonatomically, but as the owner of the lock we know that nobody
> else can write them).

Exactly.  I just did a locked add variant, and while the code looks a
little simpler, it definitely has more actual complexity to analyze.

    J
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