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/** BEGIN COPYRIGHT BLOCK
* Copyright 2001 Sun Microsystems, Inc.
* Portions copyright 1999, 2001-2003 Netscape Communications Corporation.
* All rights reserved.
* END COPYRIGHT BLOCK **/
#include <stdlib.h>
#include "crit.h"
#include "rwlock.h"
/*
* rwLock.c
* Implements a shared/exclusive lock package atop the
* critical section/condition variables. It allows multiple
* shared lock holders and only one exclusive lock holder
* on a lock variable.
*
* NOTE : It currently favors writers over readers and writers
* may starve readers. It is usually preferable to allow updates
* if they are not as frequent. We may have to change this if
* the common usage pattern differs from this.
*/
typedef struct {
CRITICAL crit; /* Short term crit to synchronize lock ops */
CONDVAR readFree; /* Indicates lock is free for readers */
CONDVAR writeFree; /* Indicates lock is free for the writer */
int numReaders; /* Number of read locks held */
int write; /* Flag to indicate write lock held */
int numWriteWaiters;/* Number of threads waiting for write lock */
} rwLock_t;
/*
* rwlock_init()
* Allocate and initialize the rwlock structure and return
* to the caller.
*/
RWLOCK rwlock_Init()
{
rwLock_t *rwLockP;
rwLockP = (rwLock_t *)PERM_MALLOC(sizeof(rwLock_t));
rwLockP->numReaders = 0;
rwLockP->write = 0;
rwLockP->numWriteWaiters = 0;
rwLockP->crit = crit_init();
rwLockP->readFree = condvar_init(rwLockP->crit);
rwLockP->writeFree = condvar_init(rwLockP->crit);
return((RWLOCK)rwLockP);
}
/*
* rwlock_terminate()
* Terminate the associated condvars and critical sections
*/
void rwlock_Terminate(RWLOCK lockP)
{
rwLock_t *rwLockP = (rwLock_t *)lockP;
crit_terminate(rwLockP->crit);
condvar_terminate(rwLockP->readFree);
condvar_terminate(rwLockP->writeFree);
PERM_FREE(rwLockP);
}
/*
* rwlock_ReadLock -- Obtain a shared lock. The caller would
* block if there are writers or writeWaiters.
*/
void rwlock_ReadLock(RWLOCK lockP)
{
rwLock_t *rwLockP = (rwLock_t *)lockP;
crit_enter(rwLockP->crit);
while (rwLockP->write || rwLockP->numWriteWaiters != 0)
condvar_wait(rwLockP->readFree);
rwLockP->numReaders++;
crit_exit(rwLockP->crit);
}
/*
* rwlock_writeLock -- Obtain an exclusive lock. The caller would
* block if there are other readers or a writer.
*/
void rwlock_WriteLock(RWLOCK lockP)
{
rwLock_t *rwLockP = (rwLock_t *)lockP;
crit_enter(rwLockP->crit);
rwLockP->numWriteWaiters++;
while (rwLockP->numReaders != 0 || rwLockP->write)
condvar_wait(rwLockP->writeFree);
rwLockP->numWriteWaiters--;
rwLockP->write = 1;
crit_exit(rwLockP->crit);
}
/*
* rw_Unlock -- Releases the lock.
*/
void rwlock_Unlock(RWLOCK lockP)
{
rwLock_t *rwLockP = (rwLock_t *)lockP;
crit_enter(rwLockP->crit);
if (rwLockP->write)
rwLockP->write = 0;
else
rwLockP->numReaders--;
if (rwLockP->numReaders == 0)
if (rwLockP->numWriteWaiters != 0)
condvar_notify(rwLockP->writeFree);
else
condvar_notifyAll(rwLockP->readFree);
crit_exit(rwLockP->crit);
}
/*
* rwlock_DemoteLock -- Change an exclusive lock on the given lock
* variable into a shared lock.
*/
void rwlock_DemoteLock(RWLOCK lockP)
{
rwLock_t *rwLockP = (rwLock_t *)lockP;
crit_enter(rwLockP->crit);
rwLockP->numReaders = 1;
rwLockP->write = 0;
if (rwLockP->numWriteWaiters == 0)
condvar_notifyAll(rwLockP->readFree);
crit_exit(rwLockP->crit);
}
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