Puzzle

Difficulty: Medium · Puzzle 1 of 6 · Topic: Testbench Debug Puzzles: Why Did the Test Hang?

Regression hangs in 3/200 seeds. Last log says the sequence is blocked in finish_item(). Find the exact liveness bug and fix it without changing stimulus intent.

Code

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27

class req_seq extends uvm_sequence #(my_item);
  \`uvm_object_utils(req_seq)
  virtual task body();
    my_item req;
    repeat (100) begin
      req = my_item::type_id::create("req");
      start_item(req);
      assert(req.randomize());
      finish_item(req);
    end
  endtask
endclass

class my_driver extends uvm_driver #(my_item);
  \`uvm_component_utils(my_driver)
  virtual task run_phase(uvm_phase phase);
    my_item req;
    forever begin
      seq_item_port.get_next_item(req);
      if (req.addr[0]) begin
        continue; // bug
      end
      drive_req(req);
      seq_item_port.item_done();
    end
  endtask
endclass

Hint

Audit get_next_item/item_done pairing on every control-flow path, not just the common path.

Step-by-step solution

1) Reproduce with one seed and add driver logs around get_next_item/item_done including txn id.
2) Observe iterations where req.addr[0] is 1 and code executes continue before item_done.
3) Confirm sequencer waits forever because one request handshake is never closed.
4) Refactor to call item_done exactly once for every get_next_item regardless of branching.
5) Add an assertion or counter check that outstanding requests return to zero at end-of-test.

Answer

Answer: Bug: early continue skips item_done, so finish_item blocks. Fix: ensure one item_done per get_next_item in all branches (including error/skip branches).

Why candidates get it wrong

Teams often increase timeout first, masking the protocol-liveness bug and wasting debug cycles.

Interviewer follow-up

How would you instrument sequencer-driver latency so this class of bug is caught within 100 cycles?

Related topics

• Back to Testbench Debug Puzzles: Why Did the Test Hang?

• Scenario & Debug Puzzles section