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From: Christoph Lameter <cl@linux.com>
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To: Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [RFC PATCH v7 1/7] Restartable sequences system call
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On Wed, 3 Aug 2016, Andy Lutomirski wrote:

> > Well, a CMPXCHG without LOCK prefix isn't all that expensive on x86.
> >
> > It is however on PPC and possibly other architectures, so in name of
> > simplicity supporting only the one variant makes sense.
> >
>
> I wouldn't want to depend on CMPXCHG.  But imagine we had primitives
> that were narrower than the full abort-on-preemption primitive.
> Specifically, suppose we had abort if (actual cpu != expected_cpu ||
> *aptr != aval).  We could do things like:
>

The latency issues that are addressed by restartable sequences require
minimim instruction overhead. Lockless CMPXCHG is very important in that
area and I would not simply remove it from consideration.