[RFC 1/2] PCI: acpiphp: enable slot only if it hasn't been enabled already

[Igor Mammedov <imammedo@redhat.com>]

When SCSI_SCAN_ASYNC is enabled (either via config or via cmd line),
adding device to bus and enabling it will kick in async host scan

 scsi_scan_host+0x21/0x1f0
 virtscsi_probe+0x2dd/0x350
 ..
 driver_probe_device+0x19/0x80
 ...
 driver_probe_device+0x19/0x80
 pci_bus_add_device+0x53/0x80
 pci_bus_add_devices+0x2b/0x70
 ...

which will schedule a job for async scan. That however breaks
if there are more than one SCSI host behind bridge, since
acpiphp_check_bridge() will walk over all slots and try to
enable each of them regardless of whether they were already
enabled.
As result the bridge might be reconfigured several times
and trigger following sequence:

  [cpu 0] acpiphp_check_bridge()
  [cpu 0]   enable_slot(a)
  [cpu 0]     configure bridge
  [cpu 0]     pci_bus_add_devices() -> scsi_scan_host(a1)
  [cpu 0]   enable_slot(b)
  ...
  [cpu 1] do_scsi_scan_host(a1) <- async jib scheduled for slot a
  ...
  [cpu 0]     configure bridge <- temporaly disables bridge

and cause do_scsi_scan_host() failure.
The same race affects SHPC (but it manages to avoid hitting the race due to
1sec delay when enabling slot).
To cover case of single device hotplug (at a time) do not attempt to
enable slot that have already been enabled.

Fixes: 40613da52b13 ("PCI: acpiphp: Reassign resources on bridge if necessary")
Reported-by: Dongli Zhang <dongli.zhang@oracle.com>
Reported-by: iona Ebner <f.ebner@proxmox.com>
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
---
 drivers/pci/hotplug/acpiphp_glue.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/hotplug/acpiphp_glue.c b/drivers/pci/hotplug/acpiphp_glue.c
index 601129772b2d..6b11609927d6 100644
--- a/drivers/pci/hotplug/acpiphp_glue.c
+++ b/drivers/pci/hotplug/acpiphp_glue.c
@@ -722,7 +722,9 @@ static void acpiphp_check_bridge(struct acpiphp_bridge *bridge)
 					trim_stale_devices(dev);
 
 			/* configure all functions */
-			enable_slot(slot, true);
+			if (slot->flags != SLOT_ENABLED) {
+				enable_slot(slot, true);
+			}
 		} else {
 			disable_slot(slot);
 		}
-- 
2.39.3