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Subject: Re: Null pointer crash at find_idlest_group on db845c w/ linus/master
From:   Valentin Schneider <valentin.schneider@arm.com>
To:     John Stultz <john.stultz@linaro.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Quentin Perret <qperret@google.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Patrick Bellasi <Patrick.Bellasi@arm.com>,
        Ingo Molnar <mingo@kernel.org>,
        lkml <linux-kernel@vger.kernel.org>
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 <c61cf647-ecb6-b9fa-51f2-8efa36134757@arm.com>
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Date:   Tue, 3 Dec 2019 23:49:11 +0000
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On 03/12/2019 23:20, Valentin Schneider wrote:
> Looking at the code, I think I got it. In find_idlest_group() we do
> initialize 'idlest_sgs' (just like busiest_stat in LB) but 'idlest' is just
> NULL. The latter is dereferenced in update_pick_idlest() just for the misfit
> case, which goes boom. And I reviewed the damn thing... Bleh.
> 
> Fixup looks easy enough, lemme write one up.
> 

Wait no, that can't be right. We can only get in there if both 'group' and
'idlest' have the same group_type, which can't be true on the first pass.
So if we go through the misfit stuff, idlest *has* to be set to something.
Bah.