VLSI DV Interview Puzzles · All levels

Lost Update in Forked Counter

Compute final hits and explain why four increments do not always produce four.

Puzzle

Difficulty: Hard · Puzzle 5 of 6 · Topic: Race Condition Puzzles

Compute final hits and explain why four increments do not always produce four.

Code

systemverilog

module tb;
  int hits = 0;

  task automatic add_hit();
    int snap;
    snap = hits;
    #0 hits = snap + 1;
  endtask

  initial begin
    fork
      repeat (2) add_hit();
      repeat (2) add_hit();
    join
    $display("[%0t] hits=%0d", $time, hits);
  end
endmodule

Hint

Read-modify-write is not atomic across concurrent threads.

Step-by-step solution

diagram

1) Multiple threads read same hits snapshot before any #0 write occurs.
2) Writes happen later in inactive and overwrite each other.
3) Typical final result is 2 instead of 4 because two updates are lost.
4) Correct fix needs synchronization (semaphore/atomic handoff), not timing hacks.

Answer

Answer: Final hits is typically 2, not 4, due to concurrent stale snapshots and overwrite races.

Why candidates get it wrong

Candidates assume integer assignment is 'thread-safe' in simulation. It is not.

Interviewer follow-up

Show a semaphore-based rewrite that guarantees hits=4.

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