Intermittently my logs fill with "ClientCursor::staticYield can't unlock b/c of recursive lock ns ..."[millions of entries] . It happens on a lot of different collections but always on a findAndModify. When it's at it's worse - the lock lasts 6-7 seconds but the entire database is unresponsive during this time.
I've read that it may may be related to insufficient indexing but I've checked that with a .explain and the cursor is BtreeCursor.
The issue is similar to this SERVER-15583
but an upgrade to 2.6.6 solved his whereas I'm already on 2.6.6.