rwlock.c

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#endif

	return (ISC_R_SUCCESS);
}

isc_result_t
isc_rwlock_tryupgrade(isc_rwlock_t *rwl) {
	isc_int32_t prevcnt;

	REQUIRE(VALID_RWLOCK(rwl));

	/* Try to acquire write access. */
	prevcnt = isc_atomic_cmpxchg(&rwl->cnt_and_flag,
				     READER_INCR, WRITER_ACTIVE);
	/*
	 * There must have been no writer, and there must have been at least
	 * one reader.
	 */
	INSIST((prevcnt & WRITER_ACTIVE) == 0 &&
	       (prevcnt & ~WRITER_ACTIVE) != 0);

	if (prevcnt == READER_INCR) {
		/*
		 * We are the only reader and have been upgraded.
		 * Now jump into the head of the writer waiting queue.
		 */
		(void)isc_atomic_xadd(&rwl->write_completions, -1);
	} else
		return (ISC_R_LOCKBUSY);

	return (ISC_R_SUCCESS);
	
}

void
isc_rwlock_downgrade(isc_rwlock_t *rwl) {
	isc_int32_t prev_readers;

	REQUIRE(VALID_RWLOCK(rwl));

	/* Become an active reader. */
	prev_readers = isc_atomic_xadd(&rwl->cnt_and_flag, READER_INCR);
	/* We must have been a writer. */
	INSIST((prev_readers & WRITER_ACTIVE) != 0);

	/* Complete write */
	(void)isc_atomic_xadd(&rwl->cnt_and_flag, -WRITER_ACTIVE);
	(void)isc_atomic_xadd(&rwl->write_completions, 1);

	/* Resume other readers */
	LOCK(&rwl->lock);
	if (rwl->readers_waiting > 0)
		BROADCAST(&rwl->readable);
	UNLOCK(&rwl->lock);
}

isc_result_t
isc_rwlock_unlock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
	isc_int32_t prev_cnt;

	REQUIRE(VALID_RWLOCK(rwl));

#ifdef ISC_RWLOCK_TRACE
	print_lock(isc_msgcat_get(isc_msgcat, ISC_MSGSET_RWLOCK,
				  ISC_MSG_PREUNLOCK, "preunlock"), rwl, type);
#endif

	if (type == isc_rwlocktype_read) {
		prev_cnt = isc_atomic_xadd(&rwl->cnt_and_flag, -READER_INCR);

		/*
		 * If we're the last reader and any writers are waiting, wake
		 * them up.  We need to wake up all of them to ensure the
		 * FIFO order.
		 */
		if (prev_cnt == READER_INCR &&
		    rwl->write_completions != rwl->write_requests) {
			LOCK(&rwl->lock);
			BROADCAST(&rwl->writeable);
			UNLOCK(&rwl->lock);
		}
	} else {
		isc_boolean_t wakeup_writers = ISC_TRUE;

		/*
		 * Reset the flag, and (implicitly) tell other writers
		 * we are done.
		 */
		(void)isc_atomic_xadd(&rwl->cnt_and_flag, -WRITER_ACTIVE);
		(void)isc_atomic_xadd(&rwl->write_completions, 1);

		if (rwl->write_granted >= rwl->write_quota ||
		    rwl->write_requests == rwl->write_completions ||
		    (rwl->cnt_and_flag & ~WRITER_ACTIVE) != 0) {
			/*
			 * We have passed the write quota, no writer is
			 * waiting, or some readers are almost ready, pending
			 * possible writers.  Note that the last case can
			 * happen even if write_requests != write_completions
			 * (which means a new writer in the queue), so we need
			 * to catch the case explicitly.
			 */
			LOCK(&rwl->lock);
			if (rwl->readers_waiting > 0) {
				wakeup_writers = ISC_FALSE;
				BROADCAST(&rwl->readable);
			}
			UNLOCK(&rwl->lock);
		}

		if (rwl->write_requests != rwl->write_completions &&
		    wakeup_writers) {
			LOCK(&rwl->lock);
			BROADCAST(&rwl->writeable);
			UNLOCK(&rwl->lock);
		}
	}

#ifdef ISC_RWLOCK_TRACE
	print_lock(isc_msgcat_get(isc_msgcat, ISC_MSGSET_RWLOCK,
				  ISC_MSG_POSTUNLOCK, "postunlock"),
		   rwl, type);
#endif

	return (ISC_R_SUCCESS);
}

#else /* ISC_PLATFORM_HAVEXADD && ISC_PLATFORM_HAVECMPXCHG */

static isc_result_t
doit(isc_rwlock_t *rwl, isc_rwlocktype_t type, isc_boolean_t nonblock) {
	isc_boolean_t skip = ISC_FALSE;
	isc_boolean_t done = ISC_FALSE;
	isc_result_t result = ISC_R_SUCCESS;

	REQUIRE(VALID_RWLOCK(rwl));

	LOCK(&rwl->lock);

#ifdef ISC_RWLOCK_TRACE
	print_lock(isc_msgcat_get(isc_msgcat, ISC_MSGSET_RWLOCK,
				  ISC_MSG_PRELOCK, "prelock"), rwl, type);
#endif

	if (type == isc_rwlocktype_read) {
		if (rwl->readers_waiting != 0)
			skip = ISC_TRUE;
		while (!done) {
			if (!skip &&
			    ((rwl->active == 0 ||
			      (rwl->type == isc_rwlocktype_read &&
			       (rwl->writers_waiting == 0 ||
			        rwl->granted < rwl->read_quota)))))
			{
				rwl->type = isc_rwlocktype_read;
				rwl->active++;
				rwl->granted++;
				done = ISC_TRUE;
			} else if (nonblock) {
				result = ISC_R_LOCKBUSY;
				done = ISC_TRUE;
			} else {
				skip = ISC_FALSE;
				rwl->readers_waiting++;
				WAIT(&rwl->readable, &rwl->lock);
				rwl->readers_waiting--;
			}
		}
	} else {
		if (rwl->writers_waiting != 0)
			skip = ISC_TRUE;
		while (!done) {
			if (!skip && rwl->active == 0) {
				rwl->type = isc_rwlocktype_write;
				rwl->active = 1;
				rwl->granted++;
				done = ISC_TRUE;
			} else if (nonblock) {
				result = ISC_R_LOCKBUSY;
				done = ISC_TRUE;
			} else {
				skip = ISC_FALSE;
				rwl->writers_waiting++;
				WAIT(&rwl->writeable, &rwl->lock);
				rwl->writers_waiting--;
			}
		}
	}

#ifdef ISC_RWLOCK_TRACE
	print_lock(isc_msgcat_get(isc_msgcat, ISC_MSGSET_RWLOCK,
				  ISC_MSG_POSTLOCK, "postlock"), rwl, type);
#endif

	UNLOCK(&rwl->lock);

	return (result);
}

isc_result_t
isc_rwlock_lock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
	return (doit(rwl, type, ISC_FALSE));
}

isc_result_t
isc_rwlock_trylock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
	return (doit(rwl, type, ISC_TRUE));
}

isc_result_t
isc_rwlock_tryupgrade(isc_rwlock_t *rwl) {
	isc_result_t result = ISC_R_SUCCESS;

	REQUIRE(VALID_RWLOCK(rwl));
	LOCK(&rwl->lock);
	REQUIRE(rwl->type == isc_rwlocktype_read);
	REQUIRE(rwl->active != 0);

	/* If we are the only reader then succeed. */
	if (rwl->active == 1) {
		rwl->original = (rwl->original == isc_rwlocktype_none) ?
				isc_rwlocktype_read : isc_rwlocktype_none;
		rwl->type = isc_rwlocktype_write;
	} else
		result = ISC_R_LOCKBUSY;

	UNLOCK(&rwl->lock);
	return (result);
}

void
isc_rwlock_downgrade(isc_rwlock_t *rwl) {

	REQUIRE(VALID_RWLOCK(rwl));
	LOCK(&rwl->lock);
	REQUIRE(rwl->type == isc_rwlocktype_write);
	REQUIRE(rwl->active == 1);

	rwl->type = isc_rwlocktype_read;
	rwl->original = (rwl->original == isc_rwlocktype_none) ?
			isc_rwlocktype_write : isc_rwlocktype_none;
	/*
	 * Resume processing any read request that were blocked when
	 * we upgraded.
	 */
	if (rwl->original == isc_rwlocktype_none &&
	    (rwl->writers_waiting == 0 || rwl->granted < rwl->read_quota) &&
	    rwl->readers_waiting > 0)
		BROADCAST(&rwl->readable);

	UNLOCK(&rwl->lock);
}

isc_result_t
isc_rwlock_unlock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {

	REQUIRE(VALID_RWLOCK(rwl));
	LOCK(&rwl->lock);
	REQUIRE(rwl->type == type);

	UNUSED(type);

#ifdef ISC_RWLOCK_TRACE
	print_lock(isc_msgcat_get(isc_msgcat, ISC_MSGSET_RWLOCK,
				  ISC_MSG_PREUNLOCK, "preunlock"), rwl, type);
#endif

	INSIST(rwl->active > 0);
	rwl->active--;
	if (rwl->active == 0) {
		if (rwl->original != isc_rwlocktype_none) {
			rwl->type = rwl->original;
			rwl->original = isc_rwlocktype_none;
		}
		if (rwl->type == isc_rwlocktype_read) {
			rwl->granted = 0;
			if (rwl->writers_waiting > 0) {
				rwl->type = isc_rwlocktype_write;
				SIGNAL(&rwl->writeable);
			} else if (rwl->readers_waiting > 0) {
				/* Does this case ever happen? */
				BROADCAST(&rwl->readable);
			}
		} else {
			if (rwl->readers_waiting > 0) {
				if (rwl->writers_waiting > 0 &&
				    rwl->granted < rwl->write_quota) {
					SIGNAL(&rwl->writeable);
				} else {
					rwl->granted = 0;
					rwl->type = isc_rwlocktype_read;
					BROADCAST(&rwl->readable);
				}
			} else if (rwl->writers_waiting > 0) {
				rwl->granted = 0;
				SIGNAL(&rwl->writeable);
			} else {
				rwl->granted = 0;
			}
		}
	}
	INSIST(rwl->original == isc_rwlocktype_none);

#ifdef ISC_RWLOCK_TRACE
	print_lock(isc_msgcat_get(isc_msgcat, ISC_MSGSET_RWLOCK,
				  ISC_MSG_POSTUNLOCK, "postunlock"),
		   rwl, type);
#endif

	UNLOCK(&rwl->lock);

	return (ISC_R_SUCCESS);
}

#endif /* ISC_PLATFORM_HAVEXADD && ISC_PLATFORM_HAVECMPXCHG */
#else /* ISC_PLATFORM_USETHREADS */

isc_result_t
isc_rwlock_init(isc_rwlock_t *rwl, unsigned int read_quota,
		unsigned int write_quota)
{
	REQUIRE(rwl != NULL);

	UNUSED(read_quota);
	UNUSED(write_quota);

	rwl->type = isc_rwlocktype_read;
	rwl->active = 0;
	rwl->magic = RWLOCK_MAGIC;

	return (ISC_R_SUCCESS);
}

isc_result_t
isc_rwlock_lock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
	REQUIRE(VALID_RWLOCK(rwl));

	if (type == isc_rwlocktype_read) {
		if (rwl->type != isc_rwlocktype_read && rwl->active != 0)
			return (ISC_R_LOCKBUSY);
		rwl->type = isc_rwlocktype_read;
		rwl->active++;
	} else {
		if (rwl->active != 0)
			return (ISC_R_LOCKBUSY);
		rwl->type = isc_rwlocktype_write;
		rwl->active = 1;
	}
        return (ISC_R_SUCCESS);
}

isc_result_t
isc_rwlock_trylock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
	return (isc_rwlock_lock(rwl, type));
}

isc_result_t
isc_rwlock_tryupgrade(isc_rwlock_t *rwl) {
	isc_result_t result = ISC_R_SUCCESS;

	REQUIRE(VALID_RWLOCK(rwl));
	REQUIRE(rwl->type == isc_rwlocktype_read);
	REQUIRE(rwl->active != 0);
	
	/* If we are the only reader then succeed. */
	if (rwl->active == 1)
		rwl->type = isc_rwlocktype_write;
	else
		result = ISC_R_LOCKBUSY;
	return (result);
}

void
isc_rwlock_downgrade(isc_rwlock_t *rwl) {

	REQUIRE(VALID_RWLOCK(rwl));
	REQUIRE(rwl->type == isc_rwlocktype_write);
	REQUIRE(rwl->active == 1);

	rwl->type = isc_rwlocktype_read;
}

isc_result_t
isc_rwlock_unlock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
	REQUIRE(VALID_RWLOCK(rwl));
	REQUIRE(rwl->type == type);

	UNUSED(type);

	INSIST(rwl->active > 0);
	rwl->active--;

	return (ISC_R_SUCCESS);
}

void
isc_rwlock_destroy(isc_rwlock_t *rwl) {
	REQUIRE(rwl != NULL);
	REQUIRE(rwl->active == 0);
	rwl->magic = 0;
}

#endif /* ISC_PLATFORM_USETHREADS */