Date:   Fri, 24 Feb 2023 05:19:18 +0100
From:   Lukas Wunner <lukas@wunner.de>
To:     Bjorn Helgaas <helgaas@kernel.org>
Cc:     Niklas Schnelle <schnelle@linux.ibm.com>,
        Gerd Bayer <gbayer@linux.ibm.com>,
        Gerald Schaefer <gerald.schaefer@linux.ibm.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        Christian Borntraeger <borntraeger@linux.ibm.com>,
        Sven Schnelle <svens@linux.ibm.com>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Pierre Morel <pmorel@linux.ibm.com>,
        Matthew Rosato <mjrosato@linux.ibm.com>,
        linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-pci@vger.kernel.org
Subject: Re: [PATCH RESEND] PCI: s390: Fix use-after-free of PCI bus
 resources with s390 per-function hotplug
Message-ID: <20230224041918.GB26067@wunner.de>
References: <1793b3cd12921b7a3fa8b3ee7e20b7cf1df1eca1.camel@linux.ibm.com>
 <20230223195345.GA3805039@bhelgaas>
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On Thu, Feb 23, 2023 at 01:53:45PM -0600, Bjorn Helgaas wrote:
> Hmm.  Good question.  Off the top of my head, I can't explain the
> difference between pci_rescan_remove_lock and pci_bus_sem, so I'm
> confused, too.  I added Lukas in case he has a ready explanation.

pci_bus_sem is a global lock which protects the "devices" list of all
pci_bus structs.

We do have a bunch of places left where the "devices" list is accessed
without holding pci_bus_sem, though I've tried to slowly eliminate
them.

pci_rescan_remove_lock is a global "big kernel lock" which serializes
any device addition and removal.

pci_rescan_remove_lock is known to be far too course-grained and thus
deadlock-prone, particularly if hotplug ports are nested (as is the
case with Thunderbolt).  It needs to be split up into several smaller
locks which protect e.g. allocation of resources of a bus (bus numbers
or MMIO / IO space) and whatever else needs to be protected.  It's just
that nobody has gotten around to identify what exactly needs to be
protected, adding the new locks and removing pci_rescan_remove_lock.

Thanks,

Lukas