Re: [PATCH -rt 2/5] Thread Migration Preemption - v2

Peter Zijlstra
2007-07-14T17:35:39Z

On Sat, 2007-07-14 at 21:16 +0400, Oleg Nesterov wrote:
> On 07/14, Peter Zijlstra wrote:
> >
> > From: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
> >
> > This patch adds the ability to protect critical sections from migration to
> > another CPU without disabling preemption.
> >
> > This will be useful to minimize the amount of preemption disabling for the -rt
> > patch. It will help leveraging improvements brought by the local_t types in
> > asm/local.h (see Documentation/local_ops.txt). Note that the updates done to
> > variables protected by migrate_disable must be either atomic or protected from
> > concurrent updates done by other threads.
> >
> > Typical use:
> >
> > migrate_disable();
> > local_inc(&__get_cpu_var(&my_local_t_var));
> > migrate_enable();
> >
> > Which will increment the variable atomically wrt the local CPU.
>
> I still think this patch is buggy. Perhaps I am wrong. Please look at
>
> http://marc.info/?l=linux-kernel&m=118417177818825
>
> there was no any reply.

Ah, yes. That would be quite nasty. I assumed the patch worked for the
non-hotplug case. But you suggest even regular wakeups could go wrong.
That would need fixing indeed.