Operating Systems

Start Lecture #10

2.3 Interprocess Communication (IPC) and Coordination/Synchronization

2.3.1 Race Conditions

A race condition occurs when two (or more) processes are about to perform some action. Depending on the exact timing, one or other goes first. If one of the processes goes first, everything works correctly, but if another one goes first, an error, possibly fatal, occurs.

Imagine two processes both accessing x, which is initially 10.

• One process is to execute x <-- x+1
• The other is to execute x <-- x-1
• When both are finished x should be 10
• But we might get 9 and might get 11!
• Show how this can happen (x <-- x+1 is not atomic)
• Tanenbaum shows how this can lead to disaster for a printer spooler

2.3.2 Critical Regions (Sections)

We must prevent interleaving sections of code that need to be atomic with respect to each other. That is, the conflicting sections need mutual exclusion. If process A is executing its critical section, it excludes process B from executing its critical section. Conversely if process B is executing is critical section, it excludes process A from executing its critical section.

Requirements for a critical section implementation.

1. No two processes may be simultaneously inside their critical section.
   
   
2. No assumption may be made about the speeds or the number of concurrent threads/processes.
   
   
3. No process outside its critical section (including the entry and exit code) may block other processes.
   
   
4. No process should have to wait forever to enter its critical section.
   • I do NOT make this last requirement.
   • I just require that the system as a whole make progress (so not all processes are blocked).
   • I refer to solutions that do not satisfy Tanenbaum's last condition as unfair, but nonetheless correct, solutions.
   • Stronger fairness conditions have also been defined.

2.3.3 Mutual exclusion with busy waiting

We will study only solutions in this class. Note that higher level solutions, e.g., having one process block when it cannot enter its critical are implemented using busy waiting algorithms.

Disabling Interrupts

The operating system can choose not to preempt itself. That is, we could choose not to preempt system processes (if the OS is client server) or processes running in system mode (if the OS is self service). Forbidding preemption for system processes would prevent the problem above where x<--x+1 not being atomic crashed the printer spooler if the spooler is part of the OS.

The way to prevent preemption of kernel-mode code is to disable interrupts. Indeed, disabling (i.e., temporarily preventing) interrupts is often done for exactly this reason.

This is not, however, sufficient for all cases.

• It does not work for user-mode programs. So the Unix print spooler, which is a user-mode program would need another solution.
• We do not want to block interrupts for too long or the system will seem unresponsive.
• Disabling interrupts is insufficient if the system has several processors.
  • The main line can be executing on both processors simultaneously so interrupts are not involved.
  • One processor cannot block interrupts on the other.

Software solutions for two processes

Lock Variables

  Initially P1wants=P2wants=false

  Code for P1                             Code for P2

  Loop forever {                          Loop forever {
     P1wants <-- true         ENTRY          P2wants <-- true
     while (P2wants) {}       ENTRY          while (P1wants) {}
     critical-section                        critical-section
     P1wants <-- false        EXIT           P2wants <-- false
     non-critical-section }                  non-critical-section }

Explain why this works.

But it is wrong!
Why?

Let's try again. The trouble was that setting want before the loop permitted us to get stuck. We had them in the wrong order!

    Initially P1wants=P2wants=false

    Code for P1                             Code for P2

    Loop forever {                          Loop forever {
       while (P2wants) {}       ENTRY          while (P1wants) {}
       P1wants <-- true         ENTRY          P2wants <-- true
       critical-section                        critical-section
       P1wants <-- false        EXIT           P2wants <-- false
       non-critical-section }                  non-critical-section }
  

Explain why this works.

But it is wrong again!
Why?