linux-kernel - Re: [PATCH 3/4] mmap locking API: Don't check locking if the mm isn't live yet

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Date:   Thu, 1 Oct 2020 20:41:27 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Jann Horn <jannh@...gle.com>
Cc:     Michel Lespinasse <walken@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>
Subject: Re: [PATCH 3/4] mmap locking API: Don't check locking if the mm
 isn't live yet

On Thu, Oct 01, 2020 at 10:16:35PM +0200, Jann Horn wrote:

> > A subclass isn't right, it has to be a _nested annotation.
> >
> > nested locking is a pretty good reason to not be able to do this, this
> > is something lockdep does struggle to model.
> 
> Did I get the terminology wrong? I thought they were the same. The
> down_*_nested() APIs take an argument "subclass", with the default
> subclass for the functions without "_nested" being 0.

AFAIK a subclass at init time sticks with the lock forever, the
_nested ones are temporary overrides.

I think what you kind of want is to start out with
lockdep_set_novalidate_class() then switch to a real class once things
are finished. Not sure exactly how :)

Jason