Conditions for Deadlock in Operating System
Last Updated : 14 Oct, 2024
A deadlock is a situation where a set of processes is blocked because each process is holding a resource and waiting for another resource acquired by some other process. In this article, we will discuss what deadlock is and the necessary conditions required for deadlock.
What is Deadlock?
Deadlock is a situation in computing where two or more processes are unable to proceed because each is waiting for the other to release resources. Key concepts include mutual exclusion, resource holding, circular wait, and no preemption. Consider a practical example when two trains are coming toward each other on the same track and there is only one track, none of the trains can move once they are in front of each other.
Deadlock is an infinite Process it means that once a process goes into deadlock it will never come out of the loop and the process will enter for an indefinite amount of time. There are only detection, resolution, and prevention techniques. But, there are no Deadlock-stopping techniques.
Deadlock How a Deadlock Can Occur?
Consider a simple scenario that includes two processes (Process A and Process B) and two resources (Resource 1 and Resource 2). Let's see that both processes begin execution at the same time.
- Process 1 obtains Resource 1.
- Process 2 obtains Resource 2.
We are currently in the following situation:
- Process 1 possesses Resource 1.
- Process 2 possesses Resource 2.
Let us now see what happens next: Process 1 requires Resource 2 to continue execution but is unable to do so because Process 2 is currently holding Resource 2. Similarly, Process 2 requires Resource 1 to continue execution but is unable to do so because Process 1 is currently holding Resource 1. Both processes are now stuck in a loop:
- Process 1 is awaiting Resource 2 from Process 2.
- Process 2 is awaiting Resource 1 from Process 1.
We have a deadlock because neither process can release the resource it is holding until it completes its task, and neither can proceed without the resource the other process is holding. Both processes are effectively "deadlocked," unable to move forward. To break the deadlock and free up resources for other processes in this situation, an external intervention, such as the operating system killing one or both processes, would be required. Deadlocks are undesirable in operating systems because they waste resources and have a negative impact on overall system performance and responsiveness. To prevent deadlocks, various resource allocation and process scheduling algorithms, such as deadlock detection and avoidance, are employed.
Necessary Conditions for the Occurrence of a Deadlock
Let's explain all four conditions related to deadlock in the context of the scenario with two processes and two resources:
- Mutual Exclusion
- Hold and Wait
- No Pre Emption
- Circular Wait
1. Mutual Exclusion
Mutual Exclusion condition requires that at least one resource be held in a non-shareable mode, which means that only one process can use the resource at any given time. Both Resource 1 and Resource 2 are non-shareable in our scenario, and only one process can have exclusive access to each resource at any given time. As an example:
- Process 1 obtains Resource 1.
- Process 2 acquires Resource 2.
2. Hold and Wait
The hold and wait condition specifies that a process must be holding at least one resource while waiting for other processes to release resources that are currently held by other processes. In our example,
- Process 1 has Resource 1 and is awaiting Resource 2.
- Process 2 currently has Resource 2 and is awaiting Resource 1.
- Both processes hold one resource while waiting for the other, satisfying the hold and wait condition.
3. No Preemption
Preemption is the act of taking a resource from a process before it has finished its task. According to the no preemption condition, resources cannot be taken forcibly from a process a process can only release resources voluntarily after completing its task.
For example – Process p1 have resource r1 and requesting for r2 that is hold by process p2. then process p1 can not preempt resource r2 until process p2 can finish his execution. After some time it try to restart by requesting both r1 and r2 resources.
Problem – This can cause the Live Lock Problem .
What is Live Lock?
Live lock is the situation where two or more processes continuously changing their state in response to each other without making any real progress. Example:
- Suppose there are two processes 1 and 2 and two resources r1 and r2.
- Now, p1 acquired r1 and need r2 & p2 acquired r2 and need r1.
- so according to above method- Both p1 and p2 detect that they can’t acquire second resource, so they release resource that they are holding and then try again.
- continuous cycle- p1 again acquired r1 and requesting to r2 p2 again acquired r2 and requesting to r1 so there is no overall progress still process are changing there state as they preempt resources and then again holding them. This the situation of Live Lock.
4. Circular Wait
Circular wait is a condition in which a set of processes are waiting for resources in such a way that there is a circular chain, with each process in the chain holding a resource that the next process needs. This is one of the necessary conditions for a deadlock to occur in a system.
Example: Imagine four processes—P1, P2, P3, and P4—and four resources—R1, R2, R3, and R4.
- P1 is holding R1 and waiting for R2 (which is held by P2).
- P2 is holding R2 and waiting for R3 (which is held by P3).
- P3 is holding R3 and waiting for R4 (which is held by P4).
- P4 is holding R4 and waiting for R1 (which is held by P1).
Circular wait exampleThis forms a circular chain where every process is waiting for a resource held by another, creating a situation where no process can proceed, leading to a deadlock.
Conclusion
Deadlocks are major problems in computers in which two or more processes remain blocked forever, each waiting for the other to release resources. A deadlock requires four conditions: mutual exclusion, hold and wait, no preemption, and circular wait. Deadlocks consume resources and reduce system performance, hence their avoidance, detection, and resolution are critical in operating systems. Deadlocks are managed and mitigated using a variety of algorithms and tactics, which ensure system stability and efficiency.
PYQs on Deadlock
Q.1: Which of the following statements is/are TRUE with respect to deadlocks? [GATE CSE 2022]
Options:
(A) The circular wait is a necessary condition for the formation of deadlock.
(B) In a system where each resource has more than one instance, a cycle in its wait-for graph indicates the presence of a deadlock.
(C) If the current allocation of resources to processes leads the system to an unsafe state, then deadlock will necessarily occur.
(D) In the resource-allocation graph of a system, if every edge is an assignment edge, then the system is not in a deadlock state.
Answer: 1 and 4
Explanation:
- Circular Wait is one of the four necessary conditions for deadlock to happen Hence it is True.
- Not necessarily; if each resource only had one instance, a cycle would have been both necessary and sufficient for a deadlock to occur. A Cycle in a multi-instance resource is required but not sufficient, and in this case, each resource is made up of more than one instance, resulting in a simple contradiction.
- An unsafe state is one in which no method of resource allocation can prevent deadlock from occurring. In contrast, in a safe state, there is a method of allocation in which all processes are completed, so the answer is False.
- In the form of request edges, the Resource Allocation graph accommodates future resource requirements. If there are no request edges, then all process resource requirements are met, so it is True.
Q.2: Suppose n processes, P1, …. Pn shares m identical resource units, which can be reserved and released one at a time. The maximum resource requirement of process Pi is Si, where Si > 0. Which one of the following is a sufficient condition for ensuring that deadlock does not occur? (GATE CS 2005)
(A) A
(B) B
(C) C
(D) D
Answer is (C) , For the solution, see Question 4 of https://www.geeksforgeeks.org/operating-systems-set-16/
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