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From:   Dan Williams <dan.j.williams@intel.com>
Date:   Sat, 23 Apr 2022 10:27:52 -0700
Message-ID: <CAPcyv4hhD5t-qm_c_=bRjbJZFg9Mjkzbvu_2MEJB87fKy3hh-g@mail.gmail.com>
Subject: Re: [PATCH v3 2/8] cxl/acpi: Add root device lockdep validation
To:     Ira Weiny <ira.weiny@intel.com>
Cc:     linux-cxl@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Waiman Long <longman@redhat.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Alison Schofield <alison.schofield@intel.com>,
        Vishal Verma <vishal.l.verma@intel.com>,
        Ben Widawsky <ben.widawsky@intel.com>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        Linux NVDIMM <nvdimm@lists.linux.dev>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Fri, Apr 22, 2022 at 5:08 PM Dan Williams <dan.j.williams@intel.com> wrote:
>
> On Fri, Apr 22, 2022 at 4:58 PM Ira Weiny <ira.weiny@intel.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.
>

So this turned out way uglier than I expected:

 drivers/cxl/acpi.c     |    4 +++-
 include/linux/device.h |   33 +++++++++++++++++++++++++--------
 2 files changed, 28 insertions(+), 9 deletions(-)

...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.