Process synchronization

Question

What is actually a problem with critical section.....please explain..

Answer 1

critical section is the part of the program where shared resources are accessed by the processes.

for example consider 2 processes and they are sharing 1 variable

p1

process1

{

       non critical section

       critical section

    non critical section

}

process2

{

      non critical section

     critical section

     non critical section

}

now,

     if a process enters into critical section in 1st program and it gets preempted and at the same time another process wants to enter in critical section then a problem occurs and that is called race around condition.

i.e, out put of a process depends on order of execution instructions of a process

to solve this problem we insert a piece of code before the critical section called entry section

and to insert another piece of code after critical section called exit section

by doing this we can avoid race around condition i.e, only 1 process enter in critical section and after completion of that process then only next process get into critical section

Comments

thanks...but what is the mutual exclusion,progress and bounded waiting criteria. . please explain

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to call any part of code as critical section it must satisify 3 conditions:

1.Mutual Exclusion: if 1 process is in the critical section then other process should not enter into critical section.

2.Progression: if no process is in critical section then no other process from outside can block it from entering the critical section.

i.e, Assume p1,p2,p3,p4 will be executing CS at some point of there process life
cycle.
At present p2 is executing CS.
p1 made a request to enter CS.
 We know that p3 and p4 will also be executing CS at some point. And at present
they are executing their reminder section.
 It should not be the case that, CS is not assigned to p1, assuming p3 and p4 will be
requiring it. If this is the case then p3 and p4 are stopping the progress of p1.
So in CS above mentioned should not happen.

3.Bounded Waiting: A bound must be exist in number of times that process are allowed to enter their critical section after process made request to enter critical section before request granted.

i.e, assume p1,p2,p3,p4 are available processes all of them require CS.
p1 made an request first.
But other processes are given access to CS because either they have higher priority
etc. In this case p1 never gets access to CS.
So p1 made an request, and p2,p3,p4 even though having higher priority cannot
enter CS more than 2 time.
This restriction will make sure p1 will get CS at some time

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thanks a lot.....