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

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Date:   Thu, 10 Aug 2017 16:23:52 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
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


* 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?

> >
> > 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,

	Ingo