VLSI DV Interview Puzzles · All levels

Semaphore Token Leak on Error Path

Two parallel sequences share one bus lock and eventually freeze under error injection. Only failing seeds hit the deadlock.

Puzzle

Difficulty: Medium · Puzzle 3 of 6 · Topic: Deadlock Scenario Puzzles: Objection and Phase Stalls

Two parallel sequences share one bus lock and eventually freeze under error injection. Only failing seeds hit the deadlock.

Code

systemverilog

semaphore bus_sem = new(1);

task send_req(my_item req);
  bus_sem.get(1);
  if (req.timeout_fault) begin
    \`uvm_error("DRV", "timeout")
    return; // bug: token never returned
  end
  drive_req(req);
  bus_sem.put(1);
endtask

Hint

Resource ownership must be balanced even in exceptional exits.

Step-by-step solution

diagram

1) Add semaphore occupancy debug counters around get/put.
2) Reproduce with timeout_fault and observe token count never returns.
3) Refactor with cleanup block so put executes for all post-get exits.
4) Consider try/finally-style coding pattern for shared resources.
5) Add assertion: semaphore token count reaches initial value at phase end.

Answer

Answer: Bug: return on timeout path leaks semaphore token, blocking all later requests. Fix: always put token after successful get, regardless of error path.

Why candidates get it wrong

People suspect simulator scheduling; this is deterministic resource leak.

Interviewer follow-up

Would a mailbox-based arbitration model be safer here than manual semaphores?

Navigate