PCI: pciehp: Avoid returning prematurely from sysfs requests

A sysfs request to enable or disable a PCIe hotplug slot should not
return before it has been carried out.  That is sought to be achieved by
waiting until the controller's "pending_events" have been cleared.

However the IRQ thread pciehp_ist() clears the "pending_events" before
it acts on them.  If pciehp_sysfs_enable_slot() / _disable_slot() happen
to check the "pending_events" after they have been cleared but while
pciehp_ist() is still running, the functions may return prematurely
with an incorrect return value.

Fix by introducing an "ist_running" flag which must be false before a sysfs
request is allowed to return.

Fixes: 32a8cef274 ("PCI: pciehp: Enable/disable exclusively from IRQ thread")
Link: https://lore.kernel.org/linux-pci/1562226638-54134-1-git-send-email-wangxiongfeng2@huawei.com
Link: https://lore.kernel.org/r/4174210466e27eb7e2243dd1d801d5f75baaffd8.1565345211.git.lukas@wunner.de
Reported-and-tested-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: stable@vger.kernel.org # v4.19+

drivers/pci/hotplug/pciehp.h

@@ -72,6 +72,7 @@ extern int pciehp_poll_time;
  * @reset_lock: prevents access to the Data Link Layer Link Active bit in the
  *	Link Status register and to the Presence Detect State bit in the Slot
  *	Status register during a slot reset which may cause them to flap
+ * @ist_running: flag to keep user request waiting while IRQ thread is running
  * @request_result: result of last user request submitted to the IRQ thread
  * @requester: wait queue to wake up on completion of user request,
  *	used for synchronous slot enable/disable request via sysfs
@@ -101,6 +102,7 @@ struct controller {
 
 	struct hotplug_slot hotplug_slot;	/* hotplug core interface */
 	struct rw_semaphore reset_lock;
+	unsigned int ist_running;
 	int request_result;
 	wait_queue_head_t requester;
 };

drivers/pci/hotplug/pciehp_ctrl.c

@@ -375,7 +375,8 @@ int pciehp_sysfs_enable_slot(struct hotplug_slot *hotplug_slot)
 		ctrl->request_result = -ENODEV;
 		pciehp_request(ctrl, PCI_EXP_SLTSTA_PDC);
 		wait_event(ctrl->requester,
-			   !atomic_read(&ctrl->pending_events));
+			   !atomic_read(&ctrl->pending_events) &&
+			   !ctrl->ist_running);
 		return ctrl->request_result;
 	case POWERON_STATE:
 		ctrl_info(ctrl, "Slot(%s): Already in powering on state\n",
@@ -408,7 +409,8 @@ int pciehp_sysfs_disable_slot(struct hotplug_slot *hotplug_slot)
 		mutex_unlock(&ctrl->state_lock);
 		pciehp_request(ctrl, DISABLE_SLOT);
 		wait_event(ctrl->requester,
-			   !atomic_read(&ctrl->pending_events));
+			   !atomic_read(&ctrl->pending_events) &&
+			   !ctrl->ist_running);
 		return ctrl->request_result;
 	case POWEROFF_STATE:
 		ctrl_info(ctrl, "Slot(%s): Already in powering off state\n",

drivers/pci/hotplug/pciehp_hpc.c

@@ -583,6 +583,7 @@ static irqreturn_t pciehp_ist(int irq, void *dev_id)
 	irqreturn_t ret;
 	u32 events;
 
+	ctrl->ist_running = true;
 	pci_config_pm_runtime_get(pdev);
 
 	/* rerun pciehp_isr() if the port was inaccessible on interrupt */
@@ -629,6 +630,7 @@ static irqreturn_t pciehp_ist(int irq, void *dev_id)
 	up_read(&ctrl->reset_lock);
 
 	pci_config_pm_runtime_put(pdev);
+	ctrl->ist_running = false;
 	wake_up(&ctrl->requester);
 	return IRQ_HANDLED;
 }