Both read/write access can't be single threaded. Consider the situation:
1. move_rsgroup_servers is called
2. while #1 is happening rsgroup region is in transition (rpc thread in #1 holds monitor lock)
3. while #2 is happening meta is in transition.
Balancer tries to figure out plan for meta region tries to get monitor lock but can't. rpc thread task won't release monitor lock since rsgroup region never gets assigned. rsgroup region never gets assigned because it can't update meta with new state.
There's a good chance this can be reproduce just by moving both rsgroup and meta region onto the same RS and call move_rsgoup_servers on the same RS.
A bunch different actors will query from group affiliation so we can't have writes block reads.
In the code prior to this patch the getter methods that retrieve group information (getRSGroup, ofTable, OfServer, etc) don't require the monitor lock so the deadlock cycle is broken.
The methods that does mutations and updates to zk and hbase:rsgroup are synchronized appropriately. Point me to where the incoherence is?