#OS Gate IT 2005 Self Doubt.

Question

Original question - https://gateoverflow.in/3823/gate2005-it-62

 

Two shared resources R1 and R2 are used by processes P1 and P2. Each process has a certain priority for accessing each resource. Let Tij denote the priority of Pi for accessing  Rj. A process Pi can snatch a resource Rh from process Pj if Tik is greater than Tjk. 
Given the following :

1. T11>T21
2. T12>T22
3. T11<T21
4. T12<T22

Which of the following conditions ensures that P1 and P2 can never deadlock?

1. (I) and (IV)
2. (II) and (III)
3. (I) and (II)
4. None of the above

 

 

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Someone please explain how to interpret this line - 

A process Pi can snatch a resource Rh from process Pj if Tik is greater than Tjk

Comments

@Shaik Brother will you please check.

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Let us say that a resource R2 is currently held by process P2, If another process (say P1) has higher priority then P2 to access the resource, then the resource can be provided to P1....

I think in place of Rh it should be Rk then it will make sense!

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in the question they given the notation that

Let T_ij denote the priority of P_i for accessing  R_j.

It means P_i is need R_j .

 

one more notation, they mention

A process P_i can snatch a resource R_h from process P_j if T_ik is greater than T_jk.  

note that as  @himgta said, actually the line is printed mistakenly

A process P_i can snatch a resource R_k from process P_j if T_ik is greater than T_jk.  

it means when P_i and P_j both needs the the resource R_k at a time, there is a tie between them, for breaking that tie they announce either T_ik > T_jk , it means process P_i is prioritize than process P_j .

 

coming to the question... they are interested in never happens the DEADLOCK

if there is a tie ===> Deadlock due to we can't say either P1 or P2 granted for the particular resource

if you didn't get how it is going to deadlock....

T₁₁ and T₂₁ it means both P₁ and P₂ needs R₁ ===> allocate R₁ to P₁.

T₁₂ and T₂₂ it means both P₁ and P₂ needs R₂ ===> allocate R₂ to P₂.

for completing the process, every Process needs both R1 and R2 ===> Hence it is in deadlock

 

For what to do to avoid deadlock?

either allocates two resources to P₁ or r allocates two resources to P₂.

allocating two resources to P1

T₁₁ > T₂₁ , and T₁₂ > T₂₂ 

allocating two resources to P2

( T₁₁ < T₂₁ , and T₁₂ < T₂₂  ) = ( T₂₁ > T₁₁ , and T₂₂ > T₁₂  )

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@Shaik @himgta. Thanks a lot both of you! :)