deadlock

1.Define entry section and exit section.

The critical section problem is to design a protocol that the processes can use to cooperate. Each process must request permission to enter its critical section. The section of the code implementing this request is the entry section. The critical section is followed by an exit section. The remaining code is the remainder section.

2.Give two hardware instructions and their definitions which can be used for implementing mutual exclusion.

• TestAndSet

boolean TestAndSet (boolean &target)

{

boolean rv = target; target = true; return rv;

}

• Swap

void Swap (boolean &a, boolean &b)

{

boolean temp = a; a = b;

b = temp;

}

3.What is semaphores?

A semaphore 'S' is a synchronization tool which is an integer value that, apart from initialization, is accessed only through two standard atomic operations; wait and signal.Semaphores can be used to deal with the n-process critical section problem. It can be also used to solve various Synchronization problems.

4.Define busy waiting and spinlock.

When a process is in its critical section, any other process that tries to enter its critical section must loop continuously in the entry code. This is called as busy waiting and this type of semaphore is also called a spinlock,because the process while waiting for the lock.

5.Define deadlock.

A process requests resources; if the resources are not available at that time, the process enters a wait state. Waiting processes may never again change state, because the resources they have requested are held by other waiting processes. This situation is called a deadlock.

6.What is the sequence in which resources may be utilized?

Under normal mode of operation, a process may utilize a resource in the following sequence:

•   Request: If the request cannot be granted immediately,then the requesting process must wait until it can acquire the resource.

•   Use: The process can operate on the resource.

•   Release: The process releases the resource.

7.What are conditions under which a deadlock situation may arise?

A deadlock situation can arise if the following four conditions hold simultaneously in a system:

a.  Mutual exclusion

b.  Hold and wait

c.  No pre-emption

8.What is a resource-allocation graph?

Deadlocks can be described more precisely in terms of a directed graph called a system resource allocation graph. This graph consists of a set of vertices V and a set of edges E. The set of vertices V is partitioned into two different types of nodes; P the set consisting of all active processes in the system and R the set consisting of all resource types in the system.

9.Define request edge and assignment edge.

A directed edge from process Pi to resource type Rj is denoted by PiàRj; it signifies that process Pi requested an instance of resource type Rj and is currently waiting for that resource. A directed edge from resource type Rj to process Pi is denoted by RjàPi, it signifies that an instance of resource type has been allocated to a process Pi. A directed edge PiàRj is called a request edge. A directed edge RjàPi is called an assignment edge.

10.What are the methods for handling deadlocks?

The deadlock problem can be dealt with in one of the three ways:

a.   Use a protocol to prevent or avoid deadlocks, ensuring that the system will never enter a deadlock state.

b.  Allow the system to enter the deadlock state, detect it and then recover.

c.   Ignore the problem all together, and pretend that deadlocks never occur in the system.