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

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Message-ID: <87valvsc8v.fsf@vitty.brq.redhat.com>
Date:   Thu, 10 Aug 2017 16:56:00 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH RESEND] x86/smpboot: Unbreak CPU0 hotplug

Ingo Molnar <mingo@...nel.org> writes:

> * Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>
>> 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.
>
> Looks like a classic ABBA deadlock, due to the use of synchronize_sched() in 
> unregister_nmi_handler(), right?
>

Exactly.

>> >
>> > Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>> > ---
>> > It's been awile since my v1 submission, no comments so far. Resending.
>> 
>> Sorry, but 
>> 
>> ping?
>> 
>> I haven't received a single comment on this since the initial submission
>> on June, 26 - is it so bad? :-)
>
> So the fix looks good to me at first sight, but wanted to wait for Thomas to ack 
> it - once he gets back from vacation.
>

Thanks!

-- 
  Vitaly