linux-kernel - Re: [PATCH RFC] x86/smpboot: Unbreak CPU0 hotplug

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Message-ID: <87fuehvq4f.fsf@vitty.brq.redhat.com>
Date:   Fri, 30 Jun 2017 10:25:36 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     x86@...nel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] x86/smpboot: Unbreak CPU0 hotplug

Vitaly Kuznetsov <vkuznets@...hat.com> writes:

> A hang on CPU0 onlining after a preceding offlining is observed. Trace
> shows that CPU0 is stuck in check_tsc_sync_target() waiting for source
> CPU to run check_tsc_sync_source() but this never happens. Source CPU,
> in its turn, is stuck on synchronize_sched() which is called from
> native_cpu_up() -> do_boot_cpu() -> unregister_nmi_handler().
>
> Fix the issue by moving unregister_nmi_handler() from do_boot_cpu() to
> native_cpu_up() after cpu onlining is done.
>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
> RFC part: I tested the fix on KVM and Xen guests and it works just fine
> but CPU0 hotplug is not something I'm really familiar with, I may be
> missing some important details. I also skipped code archeology to figure
> out when things got broken.

Any comments? This can be considered as non-RFC if not :-)

-- 
  Vitaly