Language/OS - Multiplatform Resource Library/LANGUAGE OS.iso/oper_sys/presto/prest1_0.lha/src/locks.C

C/C++ Source or Header | 1991-12-11 | 4KB | 238 lines

/*
 * locks.c
 *    Definitions some derived synchro classes
 *
 */


#define _LOCKS_C

#include "presto.h"
#include "locks.h"



Lock::Lock(char *name)    : SynchroObject    (OBJ_LOCK, name)
{
    lo_owner = 0;
}

Lock::Lock(char *name, int locktype)    : SynchroObject (locktype, name)
{
    lo_owner = 0;
}


Lock::~Lock()
{
    if (lo_owner)
        error("Can't delete a held lock!");
}



//
// Threads are given ownership of the lock on ENTRY iff there
// is no current owner.  On EXIT, we just clear the owner field
// and wakeup the first guy on the queue.
// Inline version falls through to lock if the lock is already
// held.  This optimizes entry for non-occupied routines.
//
void
Lock::lock()
{
    SynchroObject::lock();
    if (lo_owner == 0)    {
        lo_owner = thisthread;
        SynchroObject::unlock();
    } else
        lock2();
}


void
Lock::unlock()
{
    SynchroObject::lock();
    if (thisthread != lo_owner)
        error("Can't release someone else's lock");

    lo_owner = 0;
    Thread *newowner = recall();    // find new lock owner
    //
    // Dont guarantee fairness
    //
    SynchroObject::unlock();
    if (newowner)
        newowner->wakeup((SynchroObject*)this);
}



//
// Secondary looping entry point for a lock
//
void
Lock::lock2()
{
    if (thisthread->flags()&TF_SCHEDULER)
        error("Can't block a scheduler thread!");

    // expect lock to be held on the way in
    for (;;)    {
        if (lo_owner == 0)
            break;
        remember(thisthread);
        SynchroObject::unlock();
        thisthread->sleep(this);
        SynchroObject::lock();
    }
    lo_owner = thisthread;
    SynchroObject::unlock();
}

void
Lock::print(ostream& s)
{
    s << "Lock:";
    SynchroObject::print(s);
    s << form(" lo_owner=0x%x", lo_owner);
}


Monitor::Monitor(char* name) :    Lock (name, OBJ_MONITOR)
{
}

Monitor::~Monitor()
{
    if (owner())
        error("can't delete a held monitor");
}

void
Monitor::print(ostream& s)
{
    s << "Monitor:";
    Lock::print(s);
}


//
// Condition variables are bound to monitors.  If you run a condition
// variable UNBOUND, and you do not guarantee that operations on the
// condition variable are atomic (locked), bad things will happen.
// Condition variables assume that they will only be referenced
// from within a monitor, so they don't bother to lock their data
// structures when referenced.
//

Condition::Condition(char *name) : SynchroObject (OBJ_CONDVAR, name)
{
    cerr << "Warning: use of unbound condition variable " << name <<
        " is not advised\n";
    co_monitor = 0;
}

Condition::Condition(Monitor* boundmon) : SynchroObject (OBJ_CONDVAR,0)
{
    co_monitor = boundmon;
}

Condition::Condition(Monitor &boundmon) : SynchroObject (OBJ_CONDVAR, 0)
{
    co_monitor = &boundmon;
}

Condition::Condition(Monitor* boundmon, char* name) : SynchroObject (OBJ_CONDVAR,name)
{
    co_monitor = boundmon;
}

Condition::Condition(Monitor& boundmon, char* name) : SynchroObject (OBJ_CONDVAR, name)
{
    co_monitor = &boundmon;
}


Condition::~Condition()
{
}


//
// Signal a condition variable.  Backlogs probably will not get
// implemented because they may have nasty overtones.
// No locking needed since we assume we are inside monitored region
//
void
Condition::broadcast()
{
    Thread    *t;

    if (!threadok())
        error("Condition broadcast by non-owning thread!");

    while (t = waitingQueue()->get())    {    // DO NOT LOCK
        t->wakeup(this);
    }
}

//
// No locking here either
//
void
Condition::signal()
{
    if (!threadok())
        error("Condition signal by non-owning thread!");
    Thread *t = waitingQueue()->get();        // DO NOT LOCK
    if (t)
        t->wakeup(this);
}

//
// Put a thread to sleep on a condition variable.
//  Get the monitor which we currently hold and exit it.
//
// We must be responsible for remembering threads which need to
// be reawoken on an event of this condition.  If we relied on
// the sleep routines, it is possible that we could wakeup someone
// up, have them start running, and signal us, yet we have not been
// remembered since our sleep call hadn't yet gotten far enough.
//
// No locking here also
//
void
Condition::wait()
{
    if (!threadok())
        error("Condition wait by non-owning thread!");
    remember(thisthread);    // will NOT lock the queue
    if (co_monitor)    {        // release the monitor
        co_monitor->exit();
    }
    thisthread->sleep(this);

    // reaquire the monitor if we gave it up
    if (co_monitor)    {
        co_monitor->entry();
    }
}



void
Condition::print(ostream& s)
{
    s << "Condition:";
    SynchroObject::print(s);
    s << " bound ";
    if  (co_monitor)
        s << co_monitor;
    else
        s << "_unbound_";
}