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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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