After some research I still hold that fsync blocks, at least on
FreeBSD. Am I missing something?

Here's the evidence:

Code from: /usr/src/sys/syscalls/vfs_syscalls

fsync(p, uap)
        struct proc *p;
        struct fsync_args /* {
                syscallarg(int) fd;
        } */ *uap;
        register struct vnode *vp;
        struct file *fp;
        vm_object_t obj;
        int error;

        if ((error = getvnode(p->p_fd, SCARG(uap, fd), &fp)) != 0)
                return (error);
        vp = (struct vnode *)fp->f_data;
        vn_lock(vp, LK_EXCLUSIVE | LK_RETRY, p);
        if (VOP_GETVOBJECT(vp, &obj) == 0)
                vm_object_page_clean(obj, 0, 0, 0);
        if ((error = VOP_FSYNC(vp, fp->f_cred, MNT_WAIT, p)) == 0 &&
            vp->v_mount && (vp->v_mount->mnt_flag & MNT_SOFTDEP) &&
                error = (*bioops.io_fsync)(vp);
        VOP_UNLOCK(vp, 0, p);
        return (error);

Notice the calls to:

        vn_lock(vp, LK_EXCLUSIVE | LK_RETRY, p);
        VOP_UNLOCK(vp, 0, p);

surrounding the call to VOP_FSYNC.

>From the man pages for VOP_UNLOCK:


     VOP_LOCK(struct vnode *vp, int flags, struct proc *p);

     VOP_UNLOCK(struct vnode *vp, int flags, struct proc *p);

     VOP_ISLOCKED(struct vnode *vp, struct proc *p);

     vn_lock(struct vnode *vp, int flags, struct proc *p);

     These calls are used to serialize access to the filesystem, such as to
     prevent two writes to the same file from happening at the same time.

     The arguments are:

     vp     the vnode being locked or unlocked

     flags  One of the lock request types:

                  LK_SHARED         Shared lock
                  LK_EXCLUSIVE      Exclusive lock
                  LK_UPGRADE        Shared-to-exclusive upgrade
                  LK_EXCLUPGRADE    First shared-to-exclusive upgrade
                  LK_DOWNGRADE      Exclusive-to-shared downgrade
                  LK_RELEASE        Release any type of lock
                  LK_DRAIN          Wait for all lock activity to end

            The lock type may be or'ed with these lock flags:

                  LK_NOWAIT        Do not sleep to wait for lock
                  LK_SLEEPFAIL     Sleep, then return failure
                  LK_CANRECURSE    Allow recursive exclusive lock
                  LK_REENABLE      Lock is to be reenabled after drain
                  LK_NOPAUSE       No spinloop

            The lock type may be or'ed with these control flags:

                  LK_INTERLOCK    Specify when the caller already has a simple
                                  lock (VOP_LOCK will unlock the simple lock
                                  after getting the lock)
                  LK_RETRY        Retry until locked
                  LK_NOOBJ        Don't create object

     p      process context to use for the locks

     Kernel code should use vn_lock() to lock a vnode rather than calling
     VOP_LOCK() directly.

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