Re: [PATCH v7] x86: initialize secondary CPU only if master CPU will wait for it

[Toshi Kani <toshi.kani@hp.com>]

On Fri, 2014-06-20 at 14:23 +0200, Igor Mammedov wrote:
> Hang is observed on virtual machines during CPU hotplug,
> especially in big guests with many CPUs. (It reproducible
> more often if host is over-committed).
> 
> It happens because master CPU gives up waiting on
> secondary CPU and allows it to run wild. As result
> AP causes locking or crashing system. For example
> as described here: https://lkml.org/lkml/2014/3/6/257
> 
> If master CPU have sent STARTUP IPI successfully,
> and AP signalled to master CPU that it's ready
> to start initialization, make master CPU wait
> indefinitely till AP is onlined.
> To ensure that AP won't ever run wild, make it
> wait at early startup till master CPU confirms its
> intention to wait for AP. If AP doesn't respond in 10
> seconds, the master CPU will timeout and cancel
> AP onlining.
> 
> Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> ---
> v7:
>  - fix stuck boot with non SMP config
>  - fix stuck paravirtual Xen SMP boot with more than 1VCPU
>    and CPU hotplug
> v6:
>  - no changes
> v5:
>  - add smp_mb() after clearing cpu_initialized_mask in do_boot_cpu()
>  - add 10 sec timeout description into commit message.
> v4:
>  - move commont code in cpu_init() for x32/x64 in shared
>    helper function wait_formaster_cpu()
>  - add WARN_ON(cpumask_test_and_set_cpu(cpu, cpu_initialized_mask))
>    to wait_formaster_cpu()
> v3:
>  - leave timeouts in do_boot_cpu(), so that master CPU
>    won't hang if AP doesn't respond, use cpu_initialized_mask
>    as a way for AP to signal to master CPU that it's ready
>    to start initialzation.
> v2:
>  - ammend comment in cpu_init()
> ---
>  arch/x86/kernel/cpu/common.c |   29 ++++++++-----
>  arch/x86/kernel/smpboot.c    |   99 +++++++++++++-----------------------------
>  arch/x86/xen/smp.c           |    2 +
>  3 files changed, 51 insertions(+), 79 deletions(-)

For the changes under arch/x86/kernel (I'm not familiar with Xen):

Acked-by: Toshi Kani <toshi.kani@hp.com>

Thanks,
-Toshi