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:   Fri, 2 Oct 2020 01:55:51 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
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 Fri, Oct 2, 2020 at 1:41 AM Jason Gunthorpe <jgg@...pe.ca> wrote:
> 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 :)

Huh, is there an API that sets a *subclass* (not a class) at init
time? I don't think there is.

Anyway, I'm pretty sure I just need to use the normal _nested()
locking API. I'm still cleaning up and testing a little bit, but I'll
send it out in a short while, unless I run into unexpected trouble.
Let's continue this if necessary once there's a concrete patch to talk
about. :)