Re: [PATCH 1/2] cpuhotplug: use rw_semaphore for cpu_hotplug

Oleg Nesterov
2009-05-29T21:13:00Z

On 05/29, Andrew Morton wrote:
>
> On Fri, 29 May 2009 16:29:30 +0800
> Lai Jiangshan <laijs@cn.fujitsu.com> wrote:
>
> >
> > Current get_online_cpus()/put_online_cpus() re-implement
> > a rw_semaphore,
> > so it is converted to a real rw_semaphore in this fix.
> > It simplifies codes, and is good for read.
>
> > static struct {
> > - struct task_struct *active_writer;
> > - struct mutex lock; /* Synchronizes accesses to refcount, */
> > /*
> > - * Also blocks the new readers during
> > - * an ongoing cpu hotplug operation.
> > + * active_writer makes get_online_cpus()/put_online_cpus() are allowd
> > + * to be nested in cpu_hotplug_begin()/cpu_hotplug_done().
> > + *
> > + * Thus, get_online_cpus()/put_online_cpus() can be called in
> > + * CPU notifiers.
> > */
> > - int refcount;
> > + struct task_struct *active_writer;
> > + struct rw_semaphore rwlock;
> > } cpu_hotplug;

But, afaics, down_write() blocks new readers.

This means that with this patch get_online_cpus() is not recursive, no?

Oleg.