PCI/IOV: Add PCI rescan-remove locking when enabling/disabling SR-IOV

Before disabling SR-IOV via config space accesses to the parent PF,
sriov_disable() first removes the PCI devices representing the VFs.

Since commit 9d16947 ("PCI: Add global pci_lock_rescan_remove()")
such removal operations are serialized against concurrent remove and
rescan using the pci_rescan_remove_lock. No such locking was ever added
in sriov_disable() however. In particular when commit 18f9e9d
("PCI/IOV: Factor out sriov_add_vfs()") factored out the PCI device
removal into sriov_del_vfs() there was still no locking around the
pci_iov_remove_virtfn() calls.

On s390 the lack of serialization in sriov_disable() may cause double
remove and list corruption with the below (amended) trace being observed:

  PSW:  0704c00180000000 0000000c914e4b38 (klist_put+56)
  GPRS: 000003800313fb48 0000000000000000 0000000100000001 0000000000000001
	00000000f9b520a8 0000000000000000 0000000000002fbd 00000000f4cc9480
	0000000000000001 0000000000000000 0000000000000000 0000000180692828
	00000000818e8000 000003800313fe2c 000003800313fb20 000003800313fad8
  #0 [3800313fb20] device_del at c9158ad5c
  #1 [3800313fb88] pci_remove_bus_device at c915105ba
  #2 [3800313fbd0] pci_iov_remove_virtfn at c9152f198
  #3 [3800313fc28] zpci_iov_remove_virtfn at c90fb67c0
  #4 [3800313fc60] zpci_bus_remove_device at c90fb6104
  #5 [3800313fca0] __zpci_event_availability at c90fb3dca
  torvalds#6 [3800313fd08] chsc_process_sei_nt0 at c918fe4a2
  torvalds#7 [3800313fd60] crw_collect_info at c91905822
  torvalds#8 [3800313fe10] kthread at c90feb390
  torvalds#9 [3800313fe68] __ret_from_fork at c90f6aa64
  torvalds#10 [3800313fe98] ret_from_fork at c9194f3f2.

This is because in addition to sriov_disable() removing the VFs, the
platform also generates hot-unplug events for the VFs. This being the
reverse operation to the hotplug events generated by sriov_enable() and
handled via pdev->no_vf_scan. And while the event processing takes
pci_rescan_remove_lock and checks whether the struct pci_dev still exists,
the lack of synchronization makes this checking racy.

Other races may also be possible of course though given that this lack of
locking persisted so long observable races seem very rare. Even on s390 the
list corruption was only observed with certain devices since the platform
events are only triggered by config accesses after the removal, so as long
as the removal finished synchronously they would not race. Either way the
locking is missing so fix this by adding it to the sriov_del_vfs() helper.

Just like PCI rescan-remove, locking is also missing in sriov_add_vfs()
including for the error case where pci_stop_and_remove_bus_device() is
called without the PCI rescan-remove lock being held. Even in the non-error
case, adding new PCI devices and buses should be serialized via the PCI
rescan-remove lock. Add the necessary locking.

Fixes: 18f9e9d ("PCI/IOV: Factor out sriov_add_vfs()")
Signed-off-by: Niklas Schnelle <[email protected]>
Signed-off-by: Bjorn Helgaas <[email protected]>
Reviewed-by: Benjamin Block <[email protected]>
Reviewed-by: Farhan Ali <[email protected]>
Reviewed-by: Julian Ruess <[email protected]>
Cc: [email protected]
Link: https://patch.msgid.link/[email protected]

Author: niklas88

drivers/pci/iov.c

@@ -629,15 +629,18 @@ static int sriov_add_vfs(struct pci_dev *dev, u16 num_vfs)
 	if (dev->no_vf_scan)
 		return 0;
 
+	pci_lock_rescan_remove();
 	for (i = 0; i < num_vfs; i++) {
 		rc = pci_iov_add_virtfn(dev, i);
 		if (rc)
 			goto failed;
 	}
+	pci_unlock_rescan_remove();
 	return 0;
 failed:
 	while (i--)
 		pci_iov_remove_virtfn(dev, i);
+	pci_unlock_rescan_remove();
 
 	return rc;
 }
@@ -762,8 +765,10 @@ static void sriov_del_vfs(struct pci_dev *dev)
 	struct pci_sriov *iov = dev->sriov;
 	int i;
 
+	pci_lock_rescan_remove();
 	for (i = 0; i < iov->num_VFs; i++)
 		pci_iov_remove_virtfn(dev, i);
+	pci_unlock_rescan_remove();
 }
 
 static void sriov_disable(struct pci_dev *dev)