linux-kernel - Re: [PATCH 4/4] lockdep: make MAX_LOCK

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Message-ID: <20170929162332.GA15567@bhelgaas-glaptop.roam.corp.google.com>
Date:   Fri, 29 Sep 2017 11:23:32 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Govindarajulu Varadarajan <gvaradar@...co.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, benve@...co.com,
        bhelgaas@...gle.com, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, jlbec@...lplan.org, hch@....de,
        mingo@...hat.com
Subject: Re: [PATCH 4/4] lockdep: make MAX_LOCK_DEPTH configurable from
 Kconfig

On Thu, Sep 28, 2017 at 04:51:46PM -0700, Govindarajulu Varadarajan wrote:
> On Thu, 28 Sep 2017, Peter Zijlstra wrote:
> 
> >On Wed, Sep 27, 2017 at 02:42:20PM -0700, Govindarajulu Varadarajan wrote:
> >>Make MAX_LOCK_DEPTH configurable. It is set to 48 right now. Number of
> >>VFs under a PCI pf bus can exceed 48 and this disables lockdep.
> >>
> >>lockdep currently allows max of 63 held_locks.
> >
> >But why a config knob? Why not just raise the number to 64
> >unconditionally? And is that sufficient; you only state 48 is
> >insufficient, you don't actually state the VF limit.
> >
> 
> I did not want to change the default configuration for everyone.
> 
> I will change it 63 unconditionally in v2 and resubmit the series.

I'm not happy about having to increase MAX_LOCK_DEPTH based on a
number of VFs.  I haven't had time to look at the locking strategy
you're proposing, but it just doesn't feel right to have to take 50+
locks for one operation.

Bjorn