GATE2004-48

Question

Consider two processes $P_1$ and $P_2$ accessing the shared variables $X$ and $Y$ protected by two binary semaphores $S_X$ and $S_Y$ respectively, both initialized to 1. $P$ and $V$ denote the usual semaphore operators, where $P$ decrements the semaphore value, and $V$ increments the semaphore value. The pseudo-code of $P_1$ and $P_2$ is as follows:

$P_1$:	$P_2$:
While true do { $L_1$:…….. $L_2$:…….. X = X + 1; Y = Y -1; $V(S_X);$ $V(S_Y);$       }	While true do { $L_3$:…….. $L_4$:…….. Y = Y + 1; X = Y -1; $V(S_Y);$ $V(S_X);$

In order to avoid deadlock, the correct operators at $L_1$, $L_2$, $L_3$ and $L_4$ are respectively.

• A. $P(S_Y), P(S_X); P(S_X), P(S_Y)$

• B. $P(S_X), P(S_Y); P(S_Y), P(S_X)$

• C. $P(S_X), P(S_X); P(S_Y), P(S_Y)$

• D. $P(S_X), P(S_Y); P(S_X), P(S_Y)$

Answer 1

option A)  deadlock p1 : line1|p2:line3| p1: line2(block) |p2 :line4(block)

so here p1 want s(x) which is held by p2 and p2 want s(y) which is held by p1 ...

so its circular wait (hold and wait condition) .. so there is deadlock

option B) deadlock p1 : line 1| p2 line 3|p1: line 2(block) |p2 : line 4(block)

so here p1 wants sy which is held by p2 and p2 wants sx which is held by p1...so its circular wait (hold and wait ) so deadlock

option c) p1 :line 1|p2 :line 3| p2 line 4(block) |p1 line 2 (block) here p1 wants sx and p2 wants sy .. but both will not be release by its process p1 and p2 because there is no way to release them ...so stuck in deadlock ..

option d) p1 :line 1 |p2 : line 3(block because need sx ) |p1 line 2 |p2 : still block |p1 : execute cs then up the value of sx |p2 :line 3 line 4(block need sy)| p1 up the sy |p2 :lin4 4 and easily get cs ..

we can start from p2 also ... as I answered according only p1 ... but we get same answer ....

so option D) is correct 

Comments

@sonam how option is C is getting blocked.

this is my try:

P1:P(X) | P2:P(x) (blocked) | P1:P(Y) | P2:P(Y)(blocked) so here p1 gets executed successfully.

or

P1:P(X) | P1:P(X) | P2: P(Y) | P2:P(Y) in both cases P1 and P2 is getting access. how there is deadlock in option C pls explain?

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hey in option c) l1 p(sx) l2 p(sx) (in p1) and l3 p(sy) ,l4 p(sy) (in process p2) rt 

so how P1:P(X) | P2:P(x) (blocked) | P1:P(Y) | P2:P(Y)(blocked) this will be done ??

you did for option d)

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@all

what here we found . a condition where there is no possibilty of deadlock ...or where there is some possiblity of deadlock.

in option d if we  run p1 ( p(sx)) and then wait for p2(p(sx)) then it will lead to deadlock ?

comment on thiz approach....

Answer 2

Option (A):

$\Rightarrow$ Deadlock possible

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Option (B) . Same as Option (A). Deadlock possible.

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Option (C):

No need to think of another process to get a deadlock situation:

$\Rightarrow$ Both processes will be blocked once they start.

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Option (D) is ok ! as far as deadlock is concerned.

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Comments

@debashish

in option A there may b not deadlock

P1(L1)  -> P1(L2)->P1(L3)->P1(L4) AND

DO IT FOR P2 REPLACE P1 WITH P2 IN ABOVE . AND SEQUENCLY DO IT FOR P1 AND P2 ,  WILL NEVER DEADLOCK

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Ok.

Is there any possibility of deadlock ? In A ?

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??,....

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Yes, deadlock possible.

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@ mcjoshi ..yes...I think the same ...but I asked @wanted...to his comment

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@debashidh

in option A there may b not deadlock .

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QS asks to avoid options having non zero probability of deadlock. Qs does not ask which option may have deadlock.

Answer 3

let execute P1 till L1 and then execute P2 till L3..
now check options.
Option A,B,C results deadlock.
even Option C always deadlocked..

Option D works fine ..