ACPI / hotplug / PCI: Fix bridge removal race in handle_hotplug_event()

If a PCI bridge with an ACPIPHP context attached is removed via
sysfs, the code path executed as a result is the following:

pci_stop_and_remove_bus_device_locked
 pci_remove_bus
  pcibios_remove_bus
   acpi_pci_remove_bus
    acpiphp_remove_slots
     cleanup_bridge
     put_bridge
      free_bridge
       acpiphp_put_context (for each child, under context lock)
        kfree (child context)

Now, if a hotplug notify is dispatched for one of the bridge's
children and the timing is such that handle_hotplug_event() for
that notify is executed while free_bridge() above is running,
the get_bridge(context->func.parent) in handle_hotplug_event()
will not really help, because it is too late to prevent the bridge
from going away and the child's context may be freed before
hotplug_event_work() scheduled from handle_hotplug_event()
dereferences the pointer to it passed via the data argument.
That will cause a kernel crash to happpen in hotplug_event_work().

To prevent that from happening, make handle_hotplug_event()
check the is_going_away flag of the function's parent bridge
(under acpiphp_context_lock) and bail out if it's set.  Also,
make cleanup_bridge() set the bridge's is_going_away flag under
acpiphp_context_lock so that it cannot be changed between the
check and the subsequent get_bridge(context->func.parent) in
handle_hotplug_event().

Then, in the above scenario, handle_hotplug_event() will notice
that context->func.parent->is_going_away is already set and it
will exit immediately preventing the crash from happening.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/pci/hotplug/acpiphp_glue.c

@@ -441,7 +441,9 @@ static void cleanup_bridge(struct acpiphp_bridge *bridge)
 	list_del(&bridge->list);
 	mutex_unlock(&bridge_mutex);
 
+	mutex_lock(&acpiphp_context_lock);
 	bridge->is_going_away = true;
+	mutex_unlock(&acpiphp_context_lock);
 }
 
 /**
@@ -941,6 +943,7 @@ static void handle_hotplug_event(acpi_handle handle, u32 type, void *data)
 {
 	struct acpiphp_context *context;
 	u32 ost_code = ACPI_OST_SC_SUCCESS;
+	acpi_status status;
 
 	switch (type) {
 	case ACPI_NOTIFY_BUS_CHECK:
@@ -976,13 +979,20 @@ static void handle_hotplug_event(acpi_handle handle, u32 type, void *data)
 
 	mutex_lock(&acpiphp_context_lock);
 	context = acpiphp_get_context(handle);
-	if (context && !WARN_ON(context->handle != handle)) {
-		get_bridge(context->func.parent);
-		acpiphp_put_context(context);
-		acpi_hotplug_execute(hotplug_event_work, context, type);
+	if (!context || WARN_ON(context->handle != handle)
+	    || context->func.parent->is_going_away)
+		goto err_out;
+
+	get_bridge(context->func.parent);
+	acpiphp_put_context(context);
+	status = acpi_hotplug_execute(hotplug_event_work, context, type);
+	if (ACPI_SUCCESS(status)) {
 		mutex_unlock(&acpiphp_context_lock);
 		return;
 	}
+	put_bridge(context->func.parent);
+
+ err_out:
 	mutex_unlock(&acpiphp_context_lock);
 	ost_code = ACPI_OST_SC_NON_SPECIFIC_FAILURE;