Re: [RFC][PATCH] CPUSets: Move most calls to rebuild_sched_domains() to the workqueue

[Paul Menage]

On Thu, Jun 26, 2008 at 1:34 PM, Paul Menage <menage@xxxxxxxxxx> wrote:
>
> void get_online_cpus(void)
> {
>        might_sleep();
>        if (cpu_hotplug.active_writer == current)
>                return;
>        down_read(&cpu_hotplug.lock);
> }
>
> void put_online_cpus(void)
> {
>        if (cpu_hotplug.active_writer == current)
>                return;
>        up_read(&cpu_hotplug.lock);
> }
>
> static void cpu_hotplug_begin(void)
> {
>        down_write(&cpu_hotplug.lock);
>        cpu_hotplug.active_writer = current;
> }
>
> static void cpu_hotplug_done(void)
> {
>        cpu_hotplug.active_writer = NULL;
>        up_write(&cpu_hotplug.lock);
> }
>
> I think that combined with moving the async rebuild_sched_domains to a
> separate thread should solve the problem, but I'm wondering why
> cpu_hotplug.lock was implemented this way in the first place.

Oh, I guess that doesn't work because of recursive calls to
get_online_cpus(). Maybe we need a down_read_recursive() that skips
ahead of waiting writers if the lock is already held in read mode?

Paul
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