race condition

Question

if mutual conditiion holds there will no race condition

is there any case where mutual exclusion doesnt hold and there is no race condition

i want to know if mutual exclusion and race condition are dependent on each other or independent

Comments

thank you...got it :)

@shivam so i can also make the statement that race condition can occur only if the same variable is used by both process.....?

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Updating a common table, writing to a common file, changing common variable,etc leads to race conditions.

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so can we conclude that if a shared variable is not accessed in a mutually exclusive manner then race condition exists ?

Answer 1

Race condition is when output depends on the order of execution of processes.

Suppose we have two processes sharing variable:

counter = 5

P1()

{

Critical Section

if counter == 6

    exit;  

counter --;

}

P2()

{

Critical section

if counter == 4

    exit;

counter ++;

}

If p1 is executed before p2 then value of counter = 4 

If p2 is executed before p1 then value of counter = 6

We have mutual exclusion in critical section but still race condition exists.

From Wikipedia: Race condition becomes a bug when events do not happen in the order the programmer intended.

But above needed that race condition.

Comments

But here ME is not satisfied.

Consider a case, where P1 is preempted before executing counter --;

Then CPU is allocated to Process P2, now P2 is also preempted before executing counter++;

Now, both the process can manipulate the value of counter, or we can say, both can access the Critical section at the same time, so ME is not satisfied.

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Actually it is like this bro:

Entry Section

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Below code is critical section

if counter == 6

    exit;  

counter --;

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Exit Section 

Now you can use any Lock mechanism for entry and exit section