When the OplogSlotReserver reserves oplog slots here, it is not necessarily holding any locks. This could result in illegal usage – it's a requirement to hold a Global IX lock when reserving oplog slots in order to conflict with shutdown and recover to timestamp. We do typically hold locks when the oplog slots are reserved, but the locker is stashed in a SideTransactionBlock here, and the locks can be released before we release the oplog slot, e.g. here. Thus the OplogSlotReserver should acquire its own Global IX lock.