Currently, step down calls yieldLocksForPreparedTransactions by holding both RSTL and repl mutex lock. As a result, this can deadlock with prepared txn threads that have checked out the session. Consider the below case. 

1) Thread A (txn cmd) has checked out the session.
2) Step down has acquired RSTL lock and repl mutex lock.
3) Step down calls yieldLocksForPreparedTransactions which marks the thread A as killed as it has checked out the session and its transaction state is TransactionState::kPrepared
4) Thread A tries to acquire repl mutex lock which is held by step down thread.
5) Step down waits for thread A to check in the session, so that it can check out the session and perform lock yielding of that prepared txn. But, thread A can't check in the session as it waiting for the repl mutex lock which is not interruptible.

SERVER-41317 describes a problem happened due to above scenario.