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SubjectRe: [patch 2/5] x86_64: CPU hotplug support.
On Fri, Jun 03, 2005 at 10:01:55AM +0800, Shaohua Li wrote:
> On Thu, 2005-06-02 at 16:33 -0700, Ashok Raj wrote:
> > On Thu, Jun 02, 2005 at 02:19:55PM -0600, Zwane Mwaikambo wrote:
> > > On Thu, 2 Jun 2005, Ashok Raj wrote:
> > >
> > > > @@ -445,8 +477,10 @@ void __cpuinit start_secondary(void)
> > > > /*
> > > > * Allow the master to continue.
> > > > */
> > > > + lock_ipi_call_lock();
> > > > cpu_set(smp_processor_id(), cpu_online_map);
> > > > mb();
> > > > + unlock_ipi_call_lock();
> > >
> > > What's that? Is this another smp_call_function race workaround? I thought
> > > there was an additional patch to avoid the broadcast.
> >
> > The other patch avoids sending to offline cpu's, but we read cpu_online_map
> > and clear self bit in smp_call_function. If a cpu comes online, dont we
> > want this cpu to take part in smp_call_function?
> >
> > if we dont care about this new CPU participating, and if cpu_set() is atomic
> > (for all NR_CPUS) we dont need to hold call_lock, otherwise we need to hold
> > this as well.
> If a CPU isn't online, why should it participates it? If it should
> participate it, it also might do the similar thing before set cpu
> online.

Good point. I was just trying to include the just arrived cpu, in the
set, but i can convince myself that this would be any real value to include
this newly arrived cpu in that case. I can drop it.

> Some places which really care about it such as smp_send_stop should hold
> cpucontrol semaphore to me.

panic() ends up calling smp_send_stop(), i dont think we could hold a sema
in that path if we end up calling from intr context.

probably from callers of stop_machine/restart we could add lock_cpu_hotplug
not too sure how useful that would be though.
>
> Thanks,
> Shaohua
>

--
Cheers,
Ashok Raj
- Open Source Technology Center
-
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