[PATCH -next] fork: silence a false postive warning in __mmdrop

[peterz at infradead.org]

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.