From: Lukas Wunner lukas@wunner.de
commit f5eff5591b8f9c5effd25c92c758a127765f74c1 upstream.
In 2013, commits
2e35afaefe64 ("PCI: pciehp: Add reset_slot() method") 608c388122c7 ("PCI: Add slot reset option to pci_dev_reset()")
amended PCIe hotplug to mask Presence Detect Changed events during a Secondary Bus Reset. The reset thus no longer causes gratuitous slot bringdown and bringup.
However the commits neglected to serialize reset with code paths reading slot registers. For instance, a slot bringup due to an earlier hotplug event may see the Presence Detect State bit cleared during a concurrent Secondary Bus Reset.
In 2018, commit
5b3f7b7d062b ("PCI: pciehp: Avoid slot access during reset")
retrofitted the missing locking. It introduced a reset_lock which serializes a Secondary Bus Reset with other parts of pciehp.
Unfortunately the locking turns out to be overzealous: reset_lock is held for the entire enumeration and de-enumeration of hotplugged devices, including driver binding and unbinding.
Driver binding and unbinding acquires device_lock while the reset_lock of the ancestral hotplug port is held. A concurrent Secondary Bus Reset acquires the ancestral reset_lock while already holding the device_lock. The asymmetric locking order in the two code paths can lead to AB-BA deadlocks.
Michael Haeuptle reports such deadlocks on simultaneous hot-removal and vfio release (the latter implies a Secondary Bus Reset):
pciehp_ist() # down_read(reset_lock) pciehp_handle_presence_or_link_change() pciehp_disable_slot() __pciehp_disable_slot() remove_board() pciehp_unconfigure_device() pci_stop_and_remove_bus_device() pci_stop_bus_device() pci_stop_dev() device_release_driver() device_release_driver_internal() __device_driver_lock() # device_lock()
SYS_munmap() vfio_device_fops_release() vfio_device_group_close() vfio_device_close() vfio_device_last_close() vfio_pci_core_close_device() vfio_pci_core_disable() # device_lock() __pci_reset_function_locked() pci_reset_bus_function() pci_dev_reset_slot_function() pci_reset_hotplug_slot() pciehp_reset_slot() # down_write(reset_lock)
Ian May reports the same deadlock on simultaneous hot-removal and an AER-induced Secondary Bus Reset:
aer_recover_work_func() pcie_do_recovery() aer_root_reset() pci_bus_error_reset() pci_slot_reset() pci_slot_lock() # device_lock() pci_reset_hotplug_slot() pciehp_reset_slot() # down_write(reset_lock)
Fix by releasing the reset_lock during driver binding and unbinding, thereby splitting and shrinking the critical section.
Driver binding and unbinding is protected by the device_lock() and thus serialized with a Secondary Bus Reset. There's no need to additionally protect it with the reset_lock. However, pciehp does not bind and unbind devices directly, but rather invokes PCI core functions which also perform certain enumeration and de-enumeration steps.
The reset_lock's purpose is to protect slot registers, not enumeration and de-enumeration of hotplugged devices. That would arguably be the job of the PCI core, not the PCIe hotplug driver. After all, an AER-induced Secondary Bus Reset may as well happen during boot-time enumeration of the PCI hierarchy and there's no locking to prevent that either.
Exempting *de-enumeration* from the reset_lock is relatively harmless: A concurrent Secondary Bus Reset may foil config space accesses such as PME interrupt disablement. But if the device is physically gone, those accesses are pointless anyway. If the device is physically present and only logically removed through an Attention Button press or the sysfs "power" attribute, PME interrupts as well as DMA cannot come through because pciehp_unconfigure_device() disables INTx and Bus Master bits. That's still protected by the reset_lock in the present commit.
Exempting *enumeration* from the reset_lock also has limited impact: The exempted call to pci_bus_add_device() may perform device accesses through pcibios_bus_add_device() and pci_fixup_device() which are now no longer protected from a concurrent Secondary Bus Reset. Otherwise there should be no impact.
In essence, the present commit seeks to fix the AB-BA deadlocks while still retaining a best-effort reset protection for enumeration and de-enumeration of hotplugged devices -- until a general solution is implemented in the PCI core.
Link: https://lore.kernel.org/linux-pci/CS1PR8401MB0728FC6FDAB8A35C22BD90EC95F10@C... Link: https://lore.kernel.org/linux-pci/20200615143250.438252-1-ian.may@canonical.... Link: https://lore.kernel.org/linux-pci/ce878dab-c0c4-5bd0-a725-9805a075682d@amd.c... Link: https://lore.kernel.org/linux-pci/ed831249-384a-6d35-0831-70af191e9bce@huawe... Link: https://bugzilla.kernel.org/show_bug.cgi?id=215590 Fixes: 5b3f7b7d062b ("PCI: pciehp: Avoid slot access during reset") Link: https://lore.kernel.org/r/fef2b2e9edf245c049a8c5b94743c0f74ff5008a.168119190... Reported-by: Michael Haeuptle michael.haeuptle@hpe.com Reported-by: Ian May ian.may@canonical.com Reported-by: Andrey Grodzovsky andrey2805@gmail.com Reported-by: Rahul Kumar rahul.kumar1@amd.com Reported-by: Jialin Zhang zhangjialin11@huawei.com Tested-by: Anatoli Antonovitch Anatoli.Antonovitch@amd.com Signed-off-by: Lukas Wunner lukas@wunner.de Signed-off-by: Bjorn Helgaas bhelgaas@google.com Cc: stable@vger.kernel.org # v4.19+ Cc: Dan Stein dstein@hpe.com Cc: Ashok Raj ashok.raj@intel.com Cc: Alex Michon amichon@kalrayinc.com Cc: Xiongfeng Wang wangxiongfeng2@huawei.com Cc: Alex Williamson alex.williamson@redhat.com Cc: Mika Westerberg mika.westerberg@linux.intel.com Cc: Sathyanarayanan Kuppuswamy sathyanarayanan.kuppuswamy@linux.intel.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/pci/hotplug/pciehp_pci.c | 15 +++++++++++++++ 1 file changed, 15 insertions(+)
--- a/drivers/pci/hotplug/pciehp_pci.c +++ b/drivers/pci/hotplug/pciehp_pci.c @@ -55,7 +55,14 @@ int pciehp_configure_device(struct slot
pci_assign_unassigned_bridge_resources(bridge); pcie_bus_configure_settings(parent); + + /* + * Release reset_lock during driver binding + * to avoid AB-BA deadlock with device_lock. + */ + up_read(&ctrl->reset_lock); pci_bus_add_devices(parent); + down_read_nested(&ctrl->reset_lock, ctrl->depth);
out: pci_unlock_rescan_remove(); @@ -91,7 +98,15 @@ void pciehp_unconfigure_device(struct sl pci_walk_bus(dev->subordinate, pci_dev_set_disconnected, NULL); } + + /* + * Release reset_lock during driver unbinding + * to avoid AB-BA deadlock with device_lock. + */ + up_read(&ctrl->reset_lock); pci_stop_and_remove_bus_device(dev); + down_read_nested(&ctrl->reset_lock, ctrl->depth); + /* * Ensure that no new Requests will be generated from * the device.