Drivers: hv: balloon: check if ha_region_mutex was acquired in MEM_CANCEL_ONLINE case

From: Vitaly Kuznetsov <[email protected]>

Memory notifiers are being executed in a sequential order and when one of them fails returning something different from NOTIFY_OK the remainder of the notification chain is not being executed. When a memory block is being onlined in online_pages() we do memory_notify(MEM_GOING_ONLINE, ) and if one of the notifiers in the chain fails we end up doing memory_notify(MEM_CANCEL_ONLINE, ) so it is possible for a notifier to see MEM_CANCEL_ONLINE without seeing the corresponding MEM_GOING_ONLINE event. E.g. when CONFIG_KASAN is enabled the kasan_mem_notifier() is being used to prevent memory hotplug, it returns NOTIFY_BAD for all MEM_GOING_ONLINE events. As kasan_mem_notifier() comes before the hv_memory_notifier() in the notification chain we don't see the MEM_GOING_ONLINE event and we do not take the ha_region_mutex. We, however, see the MEM_CANCEL_ONLINE event and unconditionally try to release the lock, the following is observed:

[  110.850927] =====================================
[  110.850927] [ BUG: bad unlock balance detected! ]
[  110.850927] 4.1.0-rc3_bugxxxxxxx_test_xxxx #595 Not tainted
[  110.850927] -------------------------------------
[  110.850927] systemd-udevd/920 is trying to release lock
(&dm_device.ha_region_mutex) at:
[  110.850927] [<ffffffff81acda0e>] mutex_unlock+0xe/0x10
[  110.850927] but there are no more locks to release!

At the same time we can have the ha_region_mutex taken when we get the MEM_CANCEL_ONLINE event in case one of the memory notifiers after the hv_memory_notifier() in the notification chain failed so we need to add the mutex_is_locked() check. In case of MEM_ONLINE we are always supposed to have the mutex locked.

---
 drivers/hv/hv_balloon.c |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)

index cb5b7dc..8a725cd 100644
@@ -567,7 +567,9 @@ static int hv_memory_notifier(struct notifier_block *nb, unsigned long val,
 	case MEM_ONLINE:
 		dm_device.num_pages_onlined += mem->nr_pages;
 	case MEM_CANCEL_ONLINE:
-		mutex_unlock(&dm_device.ha_region_mutex);
+		if (val == MEM_ONLINE ||
+		    mutex_is_locked(&dm_device.ha_region_mutex))
+			mutex_unlock(&dm_device.ha_region_mutex);
 		if (dm_device.ha_waiting) {
 			dm_device.ha_waiting = false;
 			complete(&dm_device.ol_waitevent);

1.7.4.1