91 lines
4.3 KiB
C
91 lines
4.3 KiB
C
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#ifndef __KERN_SYNC_MONITOR_CONDVAR_H__
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#define __KERN_SYNC_MOINTOR_CONDVAR_H__
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#include <sem.h>
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/* In [OS CONCEPT] 7.7 section, the accurate define and approximate implementation of MONITOR was introduced.
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* INTRODUCTION:
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* Monitors were invented by C. A. R. Hoare and Per Brinch Hansen, and were first implemented in Brinch Hansen's
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* Concurrent Pascal language. Generally, a monitor is a language construct and the compiler usually enforces mutual exclusion. Compare this with semaphores, which are usually an OS construct.
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* DEFNIE & CHARACTERISTIC:
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* A monitor is a collection of procedures, variables, and data structures grouped together.
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* Processes can call the monitor procedures but cannot access the internal data structures.
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* Only one process at a time may be be active in a monitor.
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* Condition variables allow for blocking and unblocking.
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* cv.wait() blocks a process.
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* The process is said to be waiting for (or waiting on) the condition variable cv.
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* cv.signal() (also called cv.notify) unblocks a process waiting for the condition variable cv.
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* When this occurs, we need to still require that only one process is active in the monitor. This can be done in several ways:
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* on some systems the old process (the one executing the signal) leaves the monitor and the new one enters
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* on some systems the signal must be the last statement executed inside the monitor.
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* on some systems the old process will block until the monitor is available again.
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* on some systems the new process (the one unblocked by the signal) will remain blocked until the monitor is available again.
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* If a condition variable is signaled with nobody waiting, the signal is lost. Compare this with semaphores, in which a signal will allow a process that executes a wait in the future to no block.
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* You should not think of a condition variable as a variable in the traditional sense.
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* It does not have a value.
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* Think of it as an object in the OOP sense.
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* It has two methods, wait and signal that manipulate the calling process.
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* IMPLEMENTATION:
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* monitor mt {
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* ----------------variable------------------
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* semaphore mutex;
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* semaphore next;
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* int next_count;
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* condvar {int count, sempahore sem} cv[N];
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* other variables in mt;
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* --------condvar wait/signal---------------
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* cond_wait (cv) {
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* cv.count ++;
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* if(mt.next_count>0)
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* signal(mt.next)
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* else
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* signal(mt.mutex);
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* wait(cv.sem);
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* cv.count --;
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* }
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*
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* cond_signal(cv) {
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* if(cv.count>0) {
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* mt.next_count ++;
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* signal(cv.sem);
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* wait(mt.next);
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* mt.next_count--;
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* }
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* }
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* --------routines in monitor---------------
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* routineA_in_mt () {
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* wait(mt.mutex);
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* ...
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* real body of routineA
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* ...
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* if(next_count>0)
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* signal(mt.next);
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* else
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* signal(mt.mutex);
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* }
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*/
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typedef struct monitor monitor_t;
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typedef struct condvar{
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semaphore_t sem; // the sem semaphore is used to down the waiting proc, and the signaling proc should up the waiting proc
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int count; // the number of waiters on condvar
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monitor_t * owner; // the owner(monitor) of this condvar
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} condvar_t;
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typedef struct monitor{
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semaphore_t mutex; // the mutex lock for going into the routines in monitor, should be initialized to 1
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semaphore_t next; // the next semaphore is used to down the signaling proc itself, and the other OR wakeuped waiting proc should wake up the sleeped signaling proc.
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int next_count; // the number of of sleeped signaling proc
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condvar_t *cv; // the condvars in monitor
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} monitor_t;
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// Initialize variables in monitor.
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void monitor_init (monitor_t *cvp, size_t num_cv);
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// Unlock one of threads waiting on the condition variable.
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void cond_signal (condvar_t *cvp);
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// Suspend calling thread on a condition variable waiting for condition atomically unlock mutex in monitor,
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// and suspends calling thread on conditional variable after waking up locks mutex.
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void cond_wait (condvar_t *cvp);
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#endif /* !__KERN_SYNC_MONITOR_CONDVAR_H__ */
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