a data set of 120m docs in one collection is being updated to add data to each document.

every few million records processed (mixed reads and writes, many fewer writes), mongos becomes unresponsive. this number has some down from 20m gradually until it now locks after only a few million.

queries against each of the shard and config mongod instance show them to be responsive to requests

queries against mongos hang indefinitely as does db.stats().

gdb stack trace (attached) shows many threads in mongo::ChunkManager::getShardsForQuery waiting to obtain mongo::rwlock::rwlock