SYNCHRONIZATION M O D E R N O P E R A T I N G S Y S T E M S R E A D 2. 3 E X C E P T A N D S P R I N G 2018

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1 SYNCHRONIZATION M O D E R N O P E R A T I N G S Y S T E M S R E A D 2. 3 E X C E P T A N D S P R I N G 2018

2 INTER-PROCESS COMMUNICATION 1. How a process pass information to another process 1. Make sure processes don t fight over resources, such as last seat in a flight control system 2. Proper sequencing between threads when dependencies are present 2,3 apply to threads, but not 1- why? 2

3 SO, WHAT COULD GO WRONG RACE CONDITIONS Two processes sending printing jobs to printer s spooler directory. Printer daemon checks spooler directory periodically, prints the file, then removes its name from the queue In: next free slot in the queue Out: next job to print In and out will be stored in a file in shared space A reads in= 7 to local variable A is suspended (quantum over) B reads in=7 B writes a filename to 7 th slot B updates in=8 Eventually A runs again and writes filename in slot 7, set in to 8 3

4 RACE CONDITIONS Situations like this, where two or more processes are reading or writing some shared data and the final result depends on who runs precisely when, are called race conditions. (predictability and consistency) Process B started using a shared variable before process A was finished with it (preemption interfered) Critical region: area of process code when a shared variable is manipulated 4

5 MUTUAL EXCLUSION In printer scenario, B is suspended until time t3 when A leaves its critical region, allowing B to enter immediately.

6 MUTUAL EXCLUSION Conditions to achieve mutual exclusion and avoid race conditions 1. No two processes may be simultaneously inside their critical regions. 2. No assumptions may be made about speeds or the number of CPUs. 3. No process running outside its critical region may block any process. 4. No process should have to wait forever to enter its critical region.

7 APPROACHES TO ACHIEVE MUTUAL EXCLUSION 1. DISABLING INTERRUPTS Disabling interrupts : a process disables interrupts before it enters its critical region and enables them after it is done. Allowing user processes to disable interrupt? What if a process disable interrupts and then did not turn them on again often a useful technique within the operating system itself but is not appropriate as a general mutual exclusion mechanism for user processes. Not applicable in multiple cores, disabling interrupts will affect the current processor only. 7

8 APPROACHES TO ACHIEVE MUTUAL EXCLUSION 2.LOCK VARIABLES A software solution done = false While not done If lock == 0 lock = 1 queue[in] = filename done = True If lock == 1 wait()

9 APPROACHES TO ACHIEVE MUTUAL EXCLUSION 3.STRICT ALTERNATION The processes take turns strictly, that is achieved by not allowing any process into the critical region twice in a row. It has to be interleaved. Continuously testing a variable until some value appears is called busy waiting. A lock that uses busy waiting is called a spin lock

10 APPROACHES TO ACHIEVE MUTUAL EXCLUSION 4.PETERSON S ALGORITHM

11 APPROACHES TO ACHIEVE MUTUAL EXCLUSION PETERSON S ALGORITHM CONT D Separate the *Critical region* - array, an entry for each process. The process indicates that it is interested However, the *turn* variable can incur conflict. If two processes update it, the first value it overwritten Comes the null statement. it will allow the one whose turn was overwritten to proceed to the critical region. And it will force the process who came second to wait in the loop- until the first one is done.

12 APPROACHES TO ACHIEVE MUTUAL EXCLUSION BUSY WAITING Peterson algorithm and the TSL instructions work but require busy waiting. Wastes CPUS time Priority inversion

13 APPROACHES TO ACHIEVE MUTUAL EXCLUSION PRIORITY INVERSION PROBLEM H and L are two processes with different priority levels H is run whenever it is ready H is waiting on IO- L gets scheduled and enters its critical region. H is ready again, so L is preempted, before exiting its critical region H is now back in running, and forever in its busy waiting loop, because L never exited its critical region

14 APPROACHES TO ACHIEVE MUTUAL EXCLUSION BLOCKING PRIMITIVES Blocking system calls. Sleep is a system call that causes the caller to block, that is, be suspended until another process wakes it up. The wakeup call has one parameter, the process to be awakened.

15 PRODUCER & CONSUMER

16 APPROACHES TO ACHIEVE MUTUAL EXCLUSION TEST AND LOCK INSTRUCTION (TSL) Hardware approach TSL RX,LOCK Reads the value of a memory location LOCK into register RX and stores a nonzero value in LOCK. The two operations are indivisible. LOCK is cleared using the typical MOV instruction The CPU executing the TSL instruction locks the memory bus to prohibit other CPUs from accessing memory until it is done. How is this different from disabling interrupts?

17 SEMAPHORES Semaphores avoid the missed wakeup in the Sleep and wakeup solution. A special type of variable with hardware support (TSL or XCHG) 0 is the locked states. Up and down operations Two usages: Mutual exclusion (Binary semaphones, mutex) Synchronization Can be used for handling interrupts.

18

19 MONITORS Suppose that the two downs in the producer s code were reversed in order, so mutex was decremented before empty instead of after it. If the buffer were completely full, the producer would block, with mutex set to 0. Consequently, the next time the consumer tried to access the buffer, it would do a down on mutex, now 0, and block too.

20 MONITORS a higher-level synchronization primitive A monitor is a collection of procedures, variables, and data structures that are all grouped together in a special kind of module or package. Processes may call the procedures in a monitor whenever they want to But they cannot directly access the monitor s internal data structures from procedures declared outside the monitor.

21 MONITORS Monitors have an important property that makes them useful for achieving mutual exclusion: only one process can be active in a monitor at any instant. It is up to the compiler to implement mutual exclusion on monitor entries, but a common way is to use a mutex or a binary semaphore. Because the compiler, not the programmer, is arranging for the mutual exclusion, it is much less likely that something will go wrong.

22 MONITORS

23 BARRIERS For a group of processes. The application behavior requires that the full group of processes proceed from one phase to another collectively. A process ( or more ) reaching the barrier, will block until all processes have reached the barrier.

24 THE SLEEPING BARBER PROBLEM

25 THE SLEEPING BARBER PROBLEM

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