# Re: \[RFC PATCH v7 1/7\] Restartable sequences system call

- From: Christoph Lameter \<hidden\>
- Date: 2016-08-03 18:31:35 
- List: linux-api
- Page: https://ratatoskr.run/linux-api/2016/08/4807588
- Thread: https://ratatoskr.run/linux-api/2016/07/4799699/t.md

```text
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.
```