Interruption of process in critical section

Question

In Priority based scheduling, I came across the problem of Priority inversion which is a higher priority process is forced to wait for the lower priority task. One possible scenario is, consider three process L,M,H with order of priority L < M < H .

L is running in CS ; H also needs to run in CS ; H waits for L to come out of CS ; M interrupts L and starts running ; M runs till completion and relinquishes control ; L resumes and starts running till the end of CS ; H enters CS and starts running.

Here, my question is, regarding the statement M interrupts L and starts running i.e., can a process executing in Critical section be interrupted or pre-empted.

Answer 1

The answer is simple and yes. If someother process with a higher priority in a preemptive system doesn't need to run in critical section, i.e. doesn't need to aquire a lock which is held by a lower priority process, then it can preempt the lower priority process regardless of what it is executing.

Even if M needs the CS, it will preempt L, run, get blocked and switched out for L to continue execution.

Answer 2

Here, my question is, regarding the statement M interrupts L and starts running i.e., can a process executing in Critical section be interrupted or pre-empted.

It depends on how the critical section is implemented.

In operating system code you will frequently find critical sections implemented where interrupts are blocked. In this kind of implementation, a process will always execute the entire critical section without interruption.

In user code that uses critical sections implemented through system services, the process invariably can be interrupted. If the were not the case a process could take over the system by putting all its code in a critical section.

You are describing one of the reasons process priorities should be consistent. Unless you are doing real time processing or background batch processing, all processes should generally have the same base priority.

The old DECUS tapes used to be filled with "fair share" applications that would lower the priory of processes with high CPU usage and that would wreak havoc with system scheduling.