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

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Message-ID: <CAPcyv4jtNgfjWLyu6MtBAjwUiqe2qEBW802AzZZeg2gZ_wU9AQ@mail.gmail.com>
Date:   Fri, 22 Apr 2022 17:08:43 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     linux-cxl@...r.kernel.org, Peter Zijlstra <peterz@...radead.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 Fri, Apr 22, 2022 at 4:58 PM Ira Weiny <ira.weiny@...el.com> wrote:
>
> On Thu, Apr 21, 2022 at 08:33:18AM -0700, Dan Williams wrote:
> > The CXL "root" device, ACPI0017, is an attach point for coordinating
> > platform level CXL resources and is the parent device for a CXL port
> > topology tree. As such it has distinct locking rules relative to other
> > CXL subsystem objects, but because it is an ACPI device the lock class
> > is established well before it is given to the cxl_acpi driver.
>
> This final sentence gave me pause because it implied that the device lock class
> was set to something other than no validate.  But I don't see that anywhere in
> the acpi code.  So given that it looks to me like ACPI is just using the
> default no validate class...

Oh, good observation. *If* ACPI had set a custom lock class then
cxl_acpi would need to be careful to restore that ACPI-specific class
and not reset it to "no validate" on exit, or skip setting its own
custom class. However, I think for generic buses like ACPI that feed
devices into other subsystems it likely has little reason to set its
own class. For safety, since device_lock_set_class() is general
purpose, I'll have it emit a debug message and fail if the class is
not "no validate" on entry.

Thanks Ira!