Fix bad percpu counter state during suspend

Hi,

I got a bug report yesterday from Laszlo Ersek <[email protected]>, in which he states that his kvm instance fails to suspend. He Laszlo bisected it down to this commit:

commit 1cf7e9c68fe84248174e998922b39e508375e7c1
Author: Jens Axboe <[email protected]>
Date: Fri Nov 1 10:52:52 2013 -0600

virtio_blk: blk-mq support

where virtio-blk is converted to use the blk-mq infrastructure. After digging a bit, it became clear that the issue was with the queue drain. blk-mq tracks queue usage in a percpu counter, which is incremented on request alloc and decremented when the request is freed. The initial hunt was for an inconsistency in blk-mq, but everything seemed fine. In fact, the counter only returned crazy values when suspend was in progress. When a CPU is unplugged, the percpu counters merges that CPU state with the general state. blk-mq takes care to register a hotcpu notifier with the appropriate priority, so we know it runs after the percpu counter notifier. However, the percpu counter notifier only merges the state when the CPU is fully gone. This leaves a state transition where the CPU going away is no longer in the online mask, yet it still holds private values. This means that in this state, percpu_counter_sum() returns invalid results, and the suspend then hangs waiting for abs(dead-cpu-value) requests to complete which of course will never happen.

Fix this by clearing the state earlier, so we never have a case where the CPU isn't in online mask but still holds private state. This bug has been there since forever, I guess we don't have a lot of users where percpu counters needs to be reliable during the suspend cycle.

index 8280a5dd1727..7dd33577b905 100644
@@ -169,7 +169,7 @@ static int percpu_counter_hotcpu_callback(struct notifier_block *nb,
 	struct percpu_counter *fbc;
 
 	compute_batch_value();
-	if (action != CPU_DEAD)
+	if (action != CPU_DEAD && action != CPU_DEAD_FROZEN)
 		return NOTIFY_OK;
 
 	cpu = (unsigned long)hcpu;

Jens Axboe