private RowLockImpl getRowLock(Lock l) {
      count.incrementAndGet();
      synchronized (lock) {
        if (usable.get()) {
          return new RowLockImpl(this, l);
        } else {
          return null;
        }
      }
    }
Get an exclusive ( write lock ) lock on a given row.
@param row Which row to lock.
@return A locked RowLock. The lock is exclusive and already aqquired.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/HRegion.java