linux-kernel - Re: [PATCH v3 2/8] cxl/acpi: Add root device lockdep validation

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Message-ID: <CAPcyv4hT7TcTxV_x1hhp8zVev21SMMUO7o2NkJw5OozjDRO4dQ@mail.gmail.com>
Date:   Mon, 25 Apr 2022 11:57:21 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ira Weiny <ira.weiny@...el.com>, linux-cxl@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Alison Schofield <alison.schofield@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Ben Widawsky <ben.widawsky@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Linux NVDIMM <nvdimm@...ts.linux.dev>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/8] cxl/acpi: Add root device lockdep validation

On Mon, Apr 25, 2022 at 9:05 AM Dan Williams <dan.j.williams@...el.com> wrote:
>
> On Mon, Apr 25, 2022 at 3:33 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Sat, Apr 23, 2022 at 10:27:52AM -0700, Dan Williams wrote:
> >
> > > ...so I'm going to drop it and just add a comment about the
> > > expectations. As Peter said there's already a multitude of ways to
> > > cause false positive / negative results with lockdep so this is just
> > > one more area where one needs to be careful and understand the lock
> > > context they might be overriding.
> >
> > One safe-guard might be to check the class you're overriding is indeed
> > __no_validate__, and WARN if not. Then the unconditional reset is
> > conistent.
> >
> > Then, if/when, that WARN ever triggers you can revisit all this.
>
> Ok, that does seem to need a dummy definition of lockdep_match_class()
> in the CONFIG_LOCKDEP=n case, but that seems worth it to me for the
> sanity check.

Thankfully the comment in lockdep.h to not define a
lockdep_match_class() for the CONFIG_LOCKDEP=n stopped me from going
that direction.