ACK: [SRU][P/N][PATCH 0/2] [UBUNTU 24.04] s390/pci: Fix immediate re-add of PCI function after remove (LP: #2114174)

[Mehmet Basaran]

Acked-by: Mehmet Basaran <mehmet.basaran at canonical.com>

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Massimiliano Pellizzer <massimiliano.pellizzer at canonical.com> writes:

> BugLink: https://bugs.launchpad.net/bugs/2114174
>
> [ Impact ]
>
> s390/pci: Fix immediate re-add of PCI function after remove
>
> A PCI function may be reserved directly after being
> deconfigured. If it subsequently returns back in the standby
> state Linux may not be able to use the new instance generating
> a kernel warning about trying to create an already existing
> sysfs file for the IOMMU.
>
> The problem occurs because the new instance of the same
> underlying device is created before the prior instance is
> completely torn down. This happens because the lifetime of the
> PCI device representation in Linux is determined by reference
> counts. A driver, the network stack, or even user-space
> (including via vfio-pci) may be holding onto the device
> represenation even after the underlying device is gone.
>
> The solution to this is twofold. Firstly allow re-using the
> pre-existing struct zpci_dev and/or struct pci_dev for the newly
> re-added instance of the underlying device up until the point
> where the struct zpci_dev is fully removed. Secondly serialize
> the addition and removal of PCI functions such that re-adding
> a new instance, after the old one is already being removed, will
> wait for the removal to finish before adding the new instance.
> This fix also builds on prior upstream work of serializing state
> transitions for PCI devices e.g. from configured to standby.
>
> [ Fix ]
>
> Backport from mainline:
> - 0d48566d4b58 s390/pci: rename lock member in struct zpci_dev
> - bcb5d6c76903 s390/pci: introduce lock to synchronize state of zpci_dev's
> - 6ee600bfbe0f s390/pci: remove hotplug slot when releasing the device
> - c4a585e952ca s390/pci: Fix potential double remove of hotplug slot
> - 42420c50c68f s390/pci: Fix missing check for zpci_create_device() error return
> - 05a2538f2b48 s390/pci: Fix duplicate pci_dev_put() in disable_slot() when PF
> has child VFs
> - d76f96332967 s390/pci: Remove redundant bus removal and disable from
> zpci_release_device()
> - 47c397844869 s390/pci: Prevent self deletion in disable_slot()
> - 4b1815a52d7e s390/pci: Allow re-add of a reserved but not yet removed device
> - 774a1fa880bc s390/pci: Serialize device addition and removal
>
> [ Test Plan ]
>
> Compile tested only.
>
> The fix will be tested further by IBM following the procedure below.
>
> The issue can be reproduced looking at the behavior of the kernel wrt to NETH
> PCI functions. In fact, IBM Z firmware temporarily reserves NETH PCI functions
> to check for pending service when the last FID of a PCHID is deconfigured. When
> nothing is pending the PCI function is immediately returned in the standby
> state, thus triggering this issue quite reliably.
>
> [ Where Problems Could Occur ]
>
> The fix affects the PCI function lifecycle management in the s390 PCI hotplug
> infrastructure, specifically the serialization and reuse logic of zpci_dev and
> pci_dev structures during rapid remove and re-add cycles. An issue with this fix
> may introduce problems such as stale or incorrectly reused device state, leading
> to improper reinitialization of PCI functions.
>
>
> -- 
> kernel-team mailing list
> kernel-team at lists.ubuntu.com
> https://lists.ubuntu.com/mailman/listinfo/kernel-team
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