Critical Section Review Problems

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Critical Section Review Problems
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Problem 1

• Thread 1
• x x1
• /
• ld r1,x
• add r1,1
• st r1,x
• /

• Thread 2
• x x1
• /
• ld r1,x
• add r1,1
• st r1,x
• /

Question Give an execution sequence where the
two threads execute concurrently and the final
value of X is one. Assume that initial value of X
is zero.
3
Problem 2

• Thread 1
• Enter_Mutex
• while (busy 1)
• busy 1
• Exit_Mutex
• busy 0

Thread 2 Enter_Mutex while (busy 1)
busy 1 Exit_Mutex busy 0
Question Give an execution sequence where both
the threads execute concurrently, and the first
thread sets busy 1 and still the second thread
goes past the while loop. busy is initialized
zero.
4
Problem 3

• Thread 1
• Enter_Mutexwhile (turn 2)
• Exit_Mutex
• turn 2

Thread 2 Enter_Mutexwhile (turn
1) Exit_Mutex turn 1
Question Which two requirements of critical
section solution are violated? Give a scenario
for violations.
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Problem 4
Question Which of the four requirements for the
critical section solution are violated?
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Problem 5
Question Which of the four requirements for the
critical section solution are violated?
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Problem 6

• TSL R, Lock TSL Test and Set LockR
  register, Lock memory location
• Atomically (atomic nothing can interfere) -
  Read Lock into R - Store a non-zero value into
  Lock

Question What should be the code to enter the
critical section? Code to exit the critical
section?