Locking the lines that must not mix

Computing · Computer Systems · ages 20-22

Name ______________________ Date ____________

How does a lost update happen?
- Both threads read before either writes
- The disk runs out of space
- The program has no variables
What three steps make up an increment?
- Read, change, and write back
- Open, print, and close
- Buy, sell, and retire
What is a critical section?
- A film review
- A faster processor
- The stretch that must not be interleaved
Which lines belong inside the lock for the balance example?
- Only the print statement
- The read, change, and write together
- The whole program forever
Balance 10, two threads each add 5, and one update is lost. Give the final balance.
Answer: ______________
Locking the whole program is the recommended fix for one race.
Circle one: True False
Locking the check together with the act fixes check-then-act races.
Circle one: True False
A thread checks balance above 10, then withdraws, but the money is gone. Both lines look right. What is wrong?
- Checks never work
- Another thread changed the value between check and act
- Balances forbid checks

Answer key

For grown-ups. Fold this page away before handing over the rest.

Both threads read before either writes · Two reads of the same value mean one write overwrites the other.
Read, change, and write back · The value is loaded, modified, then stored.
The stretch that must not be interleaved · It is the lines that break when mixed with another thread.
The read, change, and write together · Lock the smallest region that covers the full stretch.
15 · Both compute 10 plus 5, so 15 is written twice.
False · Overlocking forces needless waiting; lock the smallest region.
True · No gap remains for another thread to slip through.
Another thread changed the value between check and act · Check-then-act needs the lock across both, or the gap betrays it.