linux-kernel - Re: [PATCH 4/5] x86/mce: Fix all mce notifiers to update the mce->handled bitmask

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Message-ID: <87pneiggx8.fsf@nanos.tec.linutronix.de>
Date:   Fri, 14 Feb 2020 01:18:27 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>,
        "Luck\, Tony" <tony.luck@...el.com>
Cc:     Borislav Petkov <bp@...en8.de>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/5] x86/mce: Fix all mce notifiers to update the mce->handled bitmask

Andy Lutomirski <luto@...nel.org> writes:

> On Thu, Feb 13, 2020 at 2:19 PM Luck, Tony <tony.luck@...el.com> wrote:
>>
>> On Thu, Feb 13, 2020 at 06:03:08PM +0100, Borislav Petkov wrote:
>> > On Wed, Feb 12, 2020 at 12:46:51PM -0800, Tony Luck wrote:
>> > > If the handler took any action to log or deal with the error, set
>> > > a bit int mce->handled so that the default handler on the end of
>> > > the machine check chain can see what has been done.
>> > >
>> > > [!!! What to do about NOTIFY_STOP ... any handler that returns this
>> > > value short-circuits calling subsequent entries on the chain. In
>> > > some cases this may be the right thing to do ... but it others we
>> > > really want to keep calling other functions on the chain]
>> >
>> > Yes, we can kill that NOTIFY_STOP thing in the mce code since it is
>> > nasty.
>>
>> Well, there are places where we want to keep NOTIFY_STOP.
>
> I very very strongly disagree.

Ack. The unholy mess of cpu hotplug notifiers and the at least 50 bugs
which were unearthed by converting them to a comprehensible and
symmetric state machine have documented the insanity of notifiers
nicely.

>> 1) Default case for CEC.  We want it to "hide" the corrected error.
>>    That was one of the main goals for CEC.  We've discussed cases
>>    where CEC shouldn't hide (when internal threshold exceeded and
>>    it tries to take a page offline ... probably something related to
>>    CMCI storms ... though we didn't really come to any conclusion)
>
> Then put this logic in do_machine_check() or in some sensible place
> that it calls via some ops structure or directly.  Don't hide it in
> some incomprehensible, possibly nondeterministic place in a notifier
> chain.
>
>> 2) Errata. Perhaps a vendor/platform specific function at the head
>>    of the notify chain that weeds out errors that should never have
>>    been reported.
>
> No, do this before the notifier chain please.

Right. The amount of possible handlers is really not huge.

So having a well defined flow of explicit calls including the handling
of magic workarounds in a central place makes tons of sense.

Thanks,

        tglx