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    SubjectRe: [PATCH 0/4] arch/CPU hotplug: Add missing CPU Hotplug bits to fix nasty issues
    On Thu, 2012-03-22 at 13:13 +0100, Peter Zijlstra wrote:
    > On Thu, 2012-03-22 at 16:58 +0530, Srivatsa S. Bhat wrote:
    > > Commit 5fbd036b552f633abb394a319f7c62a5c86a9cd7 (sched: Cleanup cpu_active
    > > madness) introduced some changes that made the scheduler rely on the
    > > CPU_STARTING notifier. And hence those architectures which forgot to
    > > send out the CPU_STARTING notification will almost surely get into trouble.
    > > (Xen is one example[1]).
    > The requirement is that CPU_STARTING is ran before set_cpu_online() or
    > the open-coded equivalent -- eg. blackfin gets this wrong.
    > However, it is also required that all this happens while local IRQs are
    > still disabled, since the moment we enable IRQs interrupts can happen
    > and interrupts can cause wakeups, and wakeups need to have this state
    > set up -- blackfin, cris, m32r, mips, sh, sparc64, sparc32, um?, x86 get
    > this wrong.
    > Furthermore, all archs that use CONFIG_USE_GENERIC_SMP_HELPERS should
    > hold ipi_call_lock() over setting the cpu online -- alpha, arm, m32r,
    > mips, sh, sparc32 seem wrong.

    So for parisc, anywhere in smp_callin() should be fine. Our last act on
    CPU bringup before going into cpu_idle() is to enable interrupts.

    > Furthermore, I was pondering the scenario where a 3rd cpu IPIs the newly
    > booting cpu, I suspect we need a smp_wmb() after setting cpu_active and
    > a rmb in select_fallback_rq() before reading active.
    > All in all its a complete friggin trainwreck.

    I haven't yet actually tried git head on parisc (and I won't be able to
    until I actually get my machines back [currently being shipped across
    the atlantic]) but if there's a problem it will show up quickly: parisc
    actually uses cpu hotplug to boot its secondary cpus.


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