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Date:   Tue, 8 Sep 2020 19:58:08 +0200
From:   peterz@infradead.org
To:     Qian Cai <cai@lca.pw>
Cc:     mark.rutland@arm.com, akpm@linux-foundation.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        mpe@ellerman.id.au, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH -next] fork: silence a false postive warning in __mmdrop
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On Tue, Sep 08, 2020 at 12:50:44PM -0400, Qian Cai wrote:
> > No, you're talking nonsense. We must not free @mm when
> > 'current->active_mm == mm', never.
> 
> Yes, you are right. It still trigger this below on powerpc with today's
> linux-next by fuzzing for a while (saw a few times on recent linux-next before
> as well but so far mostly reproducible on powerpc here). Any idea?

If you can reliably reproduce this, the next thing is to trace mm_count
and figure out where it goes side-ways. I suppose we're looking for an
'extra' decrement.

Mark tried this for a while but gave up because he couldn't reliably
reproduce.