[PATCH] PCI/PME: Fix race on PME polling

[Lukas Wunner]

Since commit df17e62e5bff ("PCI: Add support for polling PME state on
suspended legacy PCI devices"), the work item pci_pme_list_scan() polls
the PME status flag of devices and wakes them up if the bit is set.

The function performs a check whether a device's upstream bridge is in
D0 for otherwise the device is inaccessible, rendering PME polling
impossible.  However the check is racy because it is performed before
polling the device.  If the upstream bridge runtime suspends to D3hot
after pci_pme_list_scan() checks its power state and before it invokes
pci_pme_wakeup(), the latter will read the PMCSR as "all ones" and
mistake it for a set PME status flag.  I am seeing this race play out as
a Thunderbolt controller going to D3cold and occasionally immediately
going to D0 again because PM polling was performed at just the wrong
time.

Avoid by checking for an "all ones" PMCSR in pci_check_pme_status().

Fixes: 58ff463396ad ("PCI PM: Add function for checking PME status of devices")
Tested-by: Mika Westerberg <mika.westerberg@xxxxxxxxxxxxxxx>
Signed-off-by: Lukas Wunner <lukas@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # v2.6.34+
Cc: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
---
 drivers/pci/pci.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 8abc843b1615..eed5db9f152f 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -1989,6 +1989,8 @@ bool pci_check_pme_status(struct pci_dev *dev)
 	pci_read_config_word(dev, pmcsr_pos, &pmcsr);
 	if (!(pmcsr & PCI_PM_CTRL_PME_STATUS))
 		return false;
+	if (pmcsr == 0xffff)
+		return false;
 
 	/* Clear PME status. */
 	pmcsr |= PCI_PM_CTRL_PME_STATUS;
-- 
2.20.1