Consider the following two threads T1 and T2 updating shared variables a and b. Initially a = b = 1.

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Question

Consider the following two threads T1 and T2 updating shared variables a and b. Initially a = b = 1. Each statement executes atomically. T1: a=a+1; b=b+1. T2: b=2*b; a=2*a. Which one of the following options lists all possible combinations of values of a and b after both T1 and T2 finish execution?

Accepted Answer

Correct Answer: (A) (a=4,b=4); (a=3,b=3); (a=4,b=3)Concept -- Thread Interleaving and Race Conditions (from Silberschatz, Galvin, Gagne, Operating System Concepts):Initial: a=1, b=1. T1: (1) a=a+1; (2) b=b+1. T2: (3) b=2*b; (4) a=2*a.All possible interleavings of 4 statements (preserving T1 and T2 order internally):Interleaving 1: 1,2,3,4 (T1 complete, then T2): a=2,b=2; b=4,a=4. Result: a=4, b=4.Interleaving 2: 3,4,1,2 (T2 complete, then T1): b=2,a=2; a=3,b=3. Result: a=3, b=3.Interleaving 3: 1,3,2,4 (a=a+1; b=2*b; b=b+1; a=2*a): a=2; b=2; b=3; a=4. Result: a=4, b=3.Interleaving 4: 1,3,4,2 (a=a+1; b=2*b; a=2*a; b=b+1): a=2; b=2; a=4; b=3. Result: a=4, b=3 (same as above).Interleaving 5: 3,1,2,4 (b=2*b; a=a+1; b=b+1; a=2*a): b=2; a=2; b=3; a=4. Result: a=4, b=3.Interleaving 6: 3,1,4,2 (b=2*b; a=a+1; a=2*a; b=b+1): b=2; a=2; a=4; b=3. Result: a=4, b=3.Distinct results: {(4,4), (3,3), (4,3)}. This matches option (A).

Consider the following two threads T1 and T2 updating shared variables a and b. Initially a = b = 1. Each statement executes atomically. T1: a=a+1; b=b+1. T2: b=2b; a=2a. Which one of the following options lists all possible combinations of values of a and b after both T1 and T2 finish execution?

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Consider the following two threads T1 and T2 updating shared variables a and b. Initially a = b = 1. Each statement executes atomically. T1: a=a+1; b=b+1. T2: b=2b; a=2a. Which one of the following options lists all possible combinations of values of a and b after both T1 and T2 finish execution?

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