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Date:   Thu, 8 Dec 2022 10:55:23 -0600
From:   Bjorn Helgaas <helgaas@kernel.org>
To:     Stephen Kitt <steve@sk2.org>
Cc:     linux-kernel@vger.kernel.org, Bjorn Helgaas <bhelgaas@google.com>,
        linux-pci@vger.kernel.org, Jan Rueth <rueth@comsys.rwth-aachen.de>
Subject: Re: [PATCH] PCI/ASPM: Call pcie_aspm_sanity_check() as late as
 possible
Message-ID: <20221208165523.GA1548682@bhelgaas>
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On Thu, Dec 08, 2022 at 09:00:17AM +0100, Stephen Kitt wrote:
> Hi Bjorn,
> 
> On Wed, 7 Dec 2022 15:56:08 -0600, Bjorn Helgaas <helgaas@kernel.org> wrote:
> > On Thu, Oct 06, 2022 at 01:59:50PM +0200, Stephen Kitt wrote:
> > > In pcie_aspm_init_link_state(), a number of checks are made to
> > > determine whether the function should proceed, before the result of
> > > the call to pcie_aspm_sanity_check() is actually used. The latter
> > > function doesn't change any state, it only reports a result, so
> > > calling it later doesn't make any difference to the state of the
> > > devices or the information we have about them. But having the call
> > > early reportedly can cause null-pointer dereferences; see
> > > https://unix.stackexchange.com/q/322337 for one example with
> > > pcie_aspm=off (this was reported in 2016, but the relevant code hasn't
> > > changed since then).  
> > 
> > Thanks, Stephen!
> > 
> > That stackexchange report doesn't have much information, but it looks
> > similar to this old report from Jan Rueth, which I'm sorry to say I
> > never got resolved:
> > 
> >   https://bugzilla.kernel.org/show_bug.cgi?id=187731
> >   https://lore.kernel.org/all/4cec62c2-218a-672b-8c12-d44e8df56aae@comsys.rwth-aachen.de/#t
> > 
> > And Jan's patch is almost identical to yours :)
> > 
> > I hope to get this resolved, but I don't have time to work on it
> > before the upcoming merge window, which will probably open Sunday.
> > And then it's holiday time, so it may be January before I get back to
> > it.  I'm just dropping the links here as breadcrumbs for picking this
> > back up.
> 
> Thanks for the update! I was somewhat bemused by the dereference here, I’m
> reassured to see I’m not the only one. Unfortunately I don’t have hardware
> which exhibits this problem,

Yeah, that's a weird thing about this.  This shouldn't be a
platform-specific thing, but both stackexchange and Jan's patch
mention IBM x3850.  

Maybe both came from a single source, or maybe there's something
deeper going on.

> I submitted the patch because it seemed
> reasonably sensible even though as you say there is probably something else
> going on here. Of course if this approach is useful, Jan’s patch should go in
> rather than mine.
> 
> Anyway, it’s been six years, so a few more weeks won’t make any difference
> ;-).
> 
> Enjoy the holiday season!
> 
> Regards,
> 
> Stephen