Function foffset_lock

freebsd/kern/vfs_vnops.c:703–747

#if OFF_MAX <= LONG_MAX
off_t
foffset_lock(struct file *fp, int flags)
{
	volatile short *flagsp;
	off_t res;
	short state;

	KASSERT((flags & FOF_OFFSET) == 0, ("FOF_OFFSET passed"));

	if ((flags & FOF_NOLOCK) != 0)
		return (atomic_load_long(&fp->f_offset));

	/*
	 * According to McKusick the vn lock was protecting f_offset here.
	 * It is now protected by the FOFFSET_LOCKED flag.
	 */
	flagsp = &fp->f_vnread_flags;
	if (atomic_cmpset_acq_16(flagsp, 0, FOFFSET_LOCKED))
		return (atomic_load_long(&fp->f_offset));

	sleepq_lock(&fp->f_vnread_flags);
	state = atomic_load_16(flagsp);
	for (;;) {
		if ((state & FOFFSET_LOCKED) == 0) {
			if (!atomic_fcmpset_acq_16(flagsp, &state,
			    FOFFSET_LOCKED))
				continue;
			break;
		}
		if ((state & FOFFSET_LOCK_WAITING) == 0) {
			if (!atomic_fcmpset_acq_16(flagsp, &state,
			    state | FOFFSET_LOCK_WAITING))
				continue;
		}
		DROP_GIANT();
		sleepq_add(&fp->f_vnread_flags, NULL, "vofflock", 0, 0);
		sleepq_wait(&fp->f_vnread_flags, PUSER -1);
		PICKUP_GIANT();
		sleepq_lock(&fp->f_vnread_flags);
		state = atomic_load_16(flagsp);
	}
	res = atomic_load_long(&fp->f_offset);
	sleepq_release(&fp->f_vnread_flags);
	return (res);
}

void
foffset_unlock(struct file *fp, off_t val, int flags)