[PATCH 3/3] PCI: hv: Add pci_destroy_slot() in pci_devices_present_work(), if necessary

[Dexuan Cui <decui@microsoft.com>]

When we hot-remove a device, usually the host sends us a PCI_EJECT message,
and a PCI_BUS_RELATIONS message with bus_rel->device_count == 0. But when
we do the quick hot-add/hot-remove test, the host may not send us the
PCI_EJECT message, if the guest has not fully finished the initialization
by sending the PCI_RESOURCES_ASSIGNED* message to the host, so it's
potentially unsafe to only depend on the pci_destroy_slot() in
hv_eject_device_work(), though create_root_hv_pci_bus() ->
hv_pci_assign_slots() is not called in this case. Note: in this case, the
host still sends the guest a PCI_BUS_RELATIONS message with
bus_rel->device_count == 0.

And, in the quick hot-add/hot-remove test, we can have such a race: before
pci_devices_present_work() -> new_pcichild_device() adds the new device
into hbus->children, we may have already received the PCI_EJECT message,
and hence the taklet handler hv_pci_onchannelcallback() may fail to find
the "hpdev" by get_pcichild_wslot(hbus, dev_message->wslot.slot), so
hv_pci_eject_device() is NOT called; later create_root_hv_pci_bus() ->
hv_pci_assign_slots() creates the slot, and the PCI_BUS_RELATIONS message
with bus_rel->device_count == 0 removes the device from hbus->children, and
we end up being unable to remove the slot in hv_pci_remove() ->
hv_pci_remove_slots().

The patch removes the slot in pci_devices_present_work() when the device
is removed. This can address the above race. Note 1:
pci_devices_present_work() and hv_eject_device_work() run in the
singled-threaded hbus->wq, so there is not a double-remove issue for the
slot. Note 2: we can't offload hv_pci_eject_device() from
hv_pci_onchannelcallback() to the workqueue, because we need
hv_pci_onchannelcallback() synchronously call hv_pci_eject_device() to
poll the channel's ringbuffer to work around the
"hangs in hv_compose_msi_msg()" issue: see
commit de0aa7b2f97d ("PCI: hv: Fix 2 hang issues in hv_compose_msi_msg()")

Fixes: a15f2c08c708 ("PCI: hv: support reporting serial number as slot information")
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Cc: stable@vger.kernel.org
---
 drivers/pci/controller/pci-hyperv.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-hyperv.c
index b489412e3502..82acd6155adf 100644
--- a/drivers/pci/controller/pci-hyperv.c
+++ b/drivers/pci/controller/pci-hyperv.c
@@ -1776,6 +1776,10 @@ static void pci_devices_present_work(struct work_struct *work)
 		hpdev = list_first_entry(&removed, struct hv_pci_dev,
 					 list_entry);
 		list_del(&hpdev->list_entry);
+
+		if (hpdev->pci_slot)
+			pci_destroy_slot(hpdev->pci_slot);
+
 		put_pcichild(hpdev);
 	}
 
-- 
2.19.1