linux-kernel - Re: Null pointer crash at find_idlest

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Message-ID: <14a8e456-1f89-0dff-ae89-61e8b6d5593b@arm.com>
Date:   Tue, 3 Dec 2019 23:49:11 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     John Stultz <john.stultz@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Quentin Perret <qperret@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Patrick Bellasi <Patrick.Bellasi@....com>,
        Ingo Molnar <mingo@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: Null pointer crash at find_idlest_group on db845c w/ linus/master

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.