linux-kernel - Re: [PATCH 2/2] lockdep: Up MAX_LOCKDEP

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Message-ID: <20171130080810.vedn2ivhurgnsu62@phenom.ffwll.local>
Date:   Thu, 30 Nov 2017 09:08:10 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Daniel Vetter <daniel.vetter@...ll.ch>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        Tejun Heo <tj@...nel.org>, Kees Cook <keescook@...omium.org>,
        Tvrtko Ursulin <tvrtko.ursulin@...el.com>,
        Marta Lofstedt <marta.lofstedt@...el.com>,
        Daniel Vetter <daniel.vetter@...el.com>
Subject: Re: [PATCH 2/2] lockdep: Up MAX_LOCKDEP_CHAINS

On Thu, Nov 30, 2017 at 08:47:18AM +0100, Peter Zijlstra wrote:
> On Wed, Nov 29, 2017 at 04:41:45PM +0100, Daniel Vetter wrote:
> > cross-release ftl
> > 
> > From Chris:
> > 
> > "Fwiw, this isn't cross-release but us reloading the module many times,
> > creating a whole host of new lockclasses. Even more fun is when the
> > module gets a slightly different address and the new lock address hashes
> > into an old lock...
> 
> Yeah, this is a known issue, just reboot.
> 
> > "I did think about a module-hook to revoke the stale lockclasses, but
> > that still leaves all the hashed chains.
> 
> Its an absolute royal pain to remove all the resources consumed by a
> module, and if you manage you then have to deal with fragmented storage
> -- that is, we need to go keep track of which entries are used.
> 
> Its a giant heap of complexity that's just not worth it.
> 
> 
> Given all that, I don't see why we should up this. Just don't reload
> modules (or better, don't use modules at all).

We use excessive amounts of module reloading to validate the failure paths
of driver load. Rebooting takes too much time. I guess we could look into
just rebinding the driver without reloading, that should take the pain off
lockdep. Meanwhile we can carry this locally.

I just included this to check whether you have any plans, thanks for
clarifying that this is not worth it from a core perspective to get fixed.
The real issue we have in CI is the one the first patch papers over.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch