Question

Answer the following question: Process p1 needs to access resources r1, r2, r3, in some order. Process p2 needs to access resources r2, r3, r4, in some order. Which of the access sequences by p1 and p2 would violate an ordered resources policy and could lead to a deadlock? P1: r1, r3, r2 P2: r2, r3, r4 P1: r1, r2, r3 P2: r2, r3, r4 P1: r2, r1, r3 P2: r3, r2, r4 P1: r3, r2, r1 P2: r3, r4, r2 Choose all the options that are true.

Answer 1

Here, r1 for p1, and r4 for p2, are independent resources needed by independent processes, so we don't need to worry about r1 and r4.

Both processes p1 and p2 require r2 and r3 as common resources, and thus a deadlock might be formed whenever any one of the following occurs :

• p1 acquires r2 first and waits for r3, and p2 acquires r3 first and waits for r2, OR
• p1 acquires r3 first and waits for r2, and p2 acquires r2 first and waits for r3

Occurrence of any of the above is a cyclical dependency and thus could lead to deadlock.

We investigate each case one by one.

i)

P1: r1, r3, r2

P2: r2, r3, r4

Here, P1 acquires r3, and waits for r2.

Also, P2 acquires r2, and waits for r3.

Deadlock possible -> YES

ii)

P1: r1, r2, r3

P2: r2, r3, r4

Here, P1 acquires r2, and waits for r3.

Also, P2 acquires r2, and waits for r3.

Deadlock possible -> NO

iii)

P1: r2, r1, r3

P2: r3, r2, r4

Here, P1 acquires r2, (then r1) and waits for r3.

Also, P2 acquires r3, and waits for r2.

Deadlock possible -> YES

iv)

P1: r3, r2, r1

P2: r3, r4, r2

Here, P1 acquires r3, and waits for r2.

Also, P2 acquires r3, (then r4) and waits for r2.

Deadlock possible -> NO

Thus, the options to select are :

1st option :

P1: r1, r3, r2

P2: r2, r3, r4

3rd option :

P1: r2, r1, r3

P2: r3, r2, r4