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Date:   Fri, 15 Apr 2022 09:53:48 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     linux-cxl@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Dave Jiang <dave.jiang@intel.com>,
        Kevin Tian <kevin.tian@intel.com>,
        Vishal L Verma <vishal.l.verma@intel.com>,
        "Schofield, Alison" <alison.schofield@intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux NVDIMM <nvdimm@lists.linux.dev>
Subject: Re: [PATCH v2 02/12] device-core: Add dev->lock_class to enable
 device_lock() lockdep validation
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On Thu, Apr 14, 2022 at 12:43:31PM -0700, Dan Williams wrote:
> On Thu, Apr 14, 2022 at 12:34 PM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Thu, Apr 14, 2022 at 10:17:13AM -0700, Dan Williams wrote:
> >
> > > One more sanity check... So in driver subsystems there are cases where
> > > a device on busA hosts a topology on busB. When that happens there's a
> > > need to set the lock class late in a driver since busA knows nothing
> > > about the locking rules of busB.
> >
> > I'll pretend I konw what you're talking about ;-)
> >
> > > Since the device has a longer lifetime than a driver when the driver
> > > exits it must set dev->mutex back to the novalidate class, otherwise
> > > it sets up a use after free of the static lock_class_key.
> >
> > I'm not following, static storage has infinite lifetime.
> 
> Not static storage in a driver module.
> 
> modprobe -r fancy_lockdep_using_driver.ko
> 
> Any use of device_lock() by the core on a device that a driver in this
> module was driving will de-reference a now invalid pointer into
> whatever memory was vmalloc'd for the module static data.

Ooh, modules (I always, conveniently, forget they exist). Yes, setting a
lock instance from the core kernel to a key that's inside a module and
then taking the module out will be 'interesting'.

Most likely you'll get a splat from lockdep when doing this.