Lock Variable | Process Synchronization

Process Synchronization-

 

Before you go through this article, make sure that you have gone through the previous article on Process Synchronization.

 

We have discussed-

• Process Synchronization provides a synchronization among the processes.
• Synchronization mechanisms allow the processes to access critical section in a synchronized manner.
• This avoids the inconsistent results.

 

Lock Variable-

 

• Lock variable is a synchronization mechanism.
• It uses a lock variable to provide the synchronization among the processes executing concurrently.
• However, it completely fails to provide the synchronization.

 

It is implemented as-

 

 

Initially, lock value is set to 0.

• Lock value = 0 means the critical section is currently vacant and no process is present inside it.
• Lock value = 1 means the critical section is currently occupied and a process is present inside it.

 

Working-

 

This synchronization mechanism is supposed to work as explained in the following scenes-

 

Scene-01:

 

• Process P₀ arrives.
• It executes the lock!=0 instruction.
• Since lock value is set to 0, so it returns value 0 to the while loop.
• The while loop condition breaks.
• It sets the lock value to 1 and enters the critical section.
• Now, even if process P₀ gets preempted in the middle, no other process can enter the critical section.
• Any other process can enter only after process P₀ completes and sets the lock value to 0.

 

Scene-02:

 

• Another process P₁ arrives.
• It executes the lock!=0 instruction.
• Since lock value is set to 1, so it returns value 1 to the while loop.
• The returned value 1 does not break the while loop condition.
• The process P₁ is trapped inside an infinite while loop.
• The while loop keeps the process P₁ busy until the lock value becomes 0 and its condition breaks.

 

Scene-03:

 

• Process P₀ comes out of the critical section and sets the lock value to 0.
• The while loop condition of process P₁ breaks.
• It sets the lock value to 1 and enters the critical section.
• Now, even if process P₁ gets preempted in the middle, no other process can enter the critical section.
• Any other process can enter only after process P₁ completes and sets the lock value to 0.

 

Failure of the Mechanism-

 

• The mechanism completely fails to provide the synchronization among the processes.
• It can not even guarantee to meet the basic criterion of mutual exclusion.

 

Also Read- Criteria For Synchronization Mechanisms

 

Explanation-

 

The occurrence of the following scenes may lead to two processes present inside the critical section at the same time-

 

Scene-01:

 

• Process P₀ arrives.
• It executes the lock!=0 instruction.
• Since lock value is set to 0, so it returns value 0 to the while loop.
• The while loop condition breaks.
• Now, process P₀ gets preempted before it sets the lock value to 1.

 

Scene-02:

 

• Another process P₁ arrives.
• It executes the lock!=0 instruction.
• Since lock value is still 0, so it returns value 0 to the while loop.
• The while loop condition breaks.
• It sets the lock value to 1 and enters the critical section.
• Now, process P₁ gets preempted in the middle of the critical section.

 

Scene-03:

 

• Process P₀ gets scheduled again.
• It resumes its execution.
• Before preemption, it had already failed the while loop condition.
• Now, it begins execution from the next instruction.
• It sets the lock value to 1 (which is already 1) and enters the critical section.

 

Thus, both the processes get to present inside the critical section at the same time.

 

Similarly,

• If there are n processes, then all of them may be present inside the critical section at the same time.
• This happens when each process gets preempted immediately after breaking the while loop condition.

 

Characteristics-

 

The characteristics of this synchronization mechanism are-

• It can be used for any number of processes.
• It is a software mechanism implemented in user mode.
• There is no support required from the operating system.
• It is a busy waiting solution which keeps the CPU busy when the process is actually waiting.
• It does not fulfill any criteria of synchronization mechanism.

 

Conclusion-

 

• The lock variable synchronization mechanism is a complete failure.
• Thus, it is never used.

 

To gain better understanding about Lock Variable,

Watch this Video Lecture

 

Next Article- Test and Set Lock | Synchronization Mechanism

 

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