Function __rw_try_upgrade_int

freebsd/kern/kern_rwlock.c:1278–1347

* Attempt to do a non-blocking upgrade from a read lock to a write * lock. This will only succeed if this thread holds a single read * lock. Returns true if the upgrade succeeded and false otherwise. */

 * lock.  Returns true if the upgrade succeeded and false otherwise.
 */
int
__rw_try_upgrade_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF)
{
	uintptr_t v, setv, tid;
	struct turnstile *ts;
	int success;

	if (SCHEDULER_STOPPED())
		return (1);

	KASSERT(rw->rw_lock != RW_DESTROYED,
	    ("rw_try_upgrade() of destroyed rwlock @ %s:%d", file, line));
	__rw_assert(&rw->rw_lock, RA_RLOCKED, file, line);

	/*
	 * Attempt to switch from one reader to a writer.  If there
	 * are any write waiters, then we will have to lock the
	 * turnstile first to prevent races with another writer
	 * calling turnstile_wait() before we have claimed this
	 * turnstile.  So, do the simple case of no waiters first.
	 */
	tid = (uintptr_t)curthread;
	success = 0;
	v = RW_READ_VALUE(rw);
	for (;;) {
		if (RW_READERS(v) > 1)
			break;
		if (!(v & RW_LOCK_WAITERS)) {
			success = atomic_fcmpset_acq_ptr(&rw->rw_lock, &v, tid);
			if (!success)
				continue;
			break;
		}

		/*
		 * Ok, we think we have waiters, so lock the turnstile.
		 */
		ts = turnstile_trywait(&rw->lock_object);
		v = RW_READ_VALUE(rw);
retry_ts:
		if (RW_READERS(v) > 1) {
			turnstile_cancel(ts);
			break;
		}
		/*
		 * Try to switch from one reader to a writer again.  This time
		 * we honor the current state of the waiters flags.
		 * If we obtain the lock with the flags set, then claim
		 * ownership of the turnstile.
		 */
		setv = tid | (v & RW_LOCK_WAITERS);
		success = atomic_fcmpset_ptr(&rw->rw_lock, &v, setv);
		if (success) {
			if (v & RW_LOCK_WAITERS)
				turnstile_claim(ts);
			else
				turnstile_cancel(ts);
			break;