PCI: pciehp: Fix infinite loop in IRQ handler upon power fault

commit 23584c1 upstream.

The Power Fault Detected bit in the Slot Status register differs from
all other hotplug events in that it is sticky:  It can only be cleared
after turning off slot power.  Per PCIe r5.0, sec. 6.7.1.8:

  If a power controller detects a main power fault on the hot-plug slot,
  it must automatically set its internal main power fault latch [...].
  The main power fault latch is cleared when software turns off power to
  the hot-plug slot.

The stickiness used to cause interrupt storms and infinite loops which
were fixed in 2009 by commits 5651c48 ("PCI pciehp: fix power fault
interrupt storm problem") and 99f0169 ("PCI: pciehp: enable
software notification on empty slots").

Unfortunately in 2020 the infinite loop issue was inadvertently
reintroduced by commit 8edf533 ("PCI: pciehp: Fix MSI interrupt
race"):  The hardirq handler pciehp_isr() clears the PFD bit until
pciehp's power_fault_detected flag is set.  That happens in the IRQ
thread pciehp_ist(), which never learns of the event because the hardirq
handler is stuck in an infinite loop.  Fix by setting the
power_fault_detected flag already in the hardirq handler.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=214989
Link: https://lore.kernel.org/linux-pci/DM8PR11MB5702255A6A92F735D90A4446868B9@DM8PR11MB5702.namprd11.prod.outlook.com
Fixes: 8edf533 ("PCI: pciehp: Fix MSI interrupt race")
Link: https://lore.kernel.org/r/66eaeef31d4997ceea357ad93259f290ededecfd.1637187226.git.lukas@wunner.de
Reported-by: Joseph Bao <joseph.bao@intel.com>
Tested-by: Joseph Bao <joseph.bao@intel.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: Stuart Hayes <stuart.w.hayes@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/pci/hotplug/pciehp_hpc.c

@@ -642,6 +642,8 @@ static irqreturn_t pciehp_isr(int irq, void *dev_id)
 	 */
 	if (ctrl->power_fault_detected)
 		status &= ~PCI_EXP_SLTSTA_PFD;
+	else if (status & PCI_EXP_SLTSTA_PFD)
+		ctrl->power_fault_detected = true;
 
 	events |= status;
 	if (!events) {
@@ -651,7 +653,7 @@ static irqreturn_t pciehp_isr(int irq, void *dev_id)
 	}
 
 	if (status) {
-		pcie_capability_write_word(pdev, PCI_EXP_SLTSTA, events);
+		pcie_capability_write_word(pdev, PCI_EXP_SLTSTA, status);
 
 		/*
 		 * In MSI mode, all event bits must be zero before the port
@@ -725,8 +727,7 @@ static irqreturn_t pciehp_ist(int irq, void *dev_id)
 	}
 
 	/* Check Power Fault Detected */
-	if ((events & PCI_EXP_SLTSTA_PFD) && !ctrl->power_fault_detected) {
-		ctrl->power_fault_detected = 1;
+	if (events & PCI_EXP_SLTSTA_PFD) {
 		ctrl_err(ctrl, "Slot(%s): Power fault\n", slot_name(ctrl));
 		pciehp_set_indicators(ctrl, PCI_EXP_SLTCTL_PWR_IND_OFF,
 				      PCI_EXP_SLTCTL_ATTN_IND_ON);