Re: 2.6.16-rc6-mm2

Andrew Morton
2006-06-13T11:46:03Z

On Tue, 13 Jun 2006 15:08:40 +1000
Keith Owens <kaos@sgi.com> wrote:

> Andi Kleen (on Tue, 13 Jun 2006 06:56:45 +0200) wrote:
> >
> >> I have previously suggested a lightweight solution that pins a process
> >> to a cpu
> >
> >That is preempt_disable()/preempt_enable() effectively
> >It's also light weight as much as these things can be.
>
> The difference being that preempt_disable() does not allow the code to
> sleep. There are some places where we want to use cpu local data and
> the code can tolerate preemption and even sleeping, as long as the
> process schedules back on the same cpu.

It would be easy to use this mechanism wrongly:

thread 1 on CPU N thread 2 on CPU N

foo = per_cpu(...)
<preempt>
foo = per_cpu(...);
foo++;
per_cpu(...) = foo;
<unpreempt>
foo++;
per_cpu(...) = foo; // whoops

In which scenarios did you envisage it being used?