Comment # 23 on bug 909912 from Thomas Blume

 (In reply to Olaf Hering from comment #21)
> (In reply to Olaf Hering from comment #20)
> 
> >   x-systemd.device-timeout=12
> 
> This option has to go into fstab.
> 
> I think what happens is that one branch processes crypttab and another
> fstab. There is no connection between both. Even if the timeout paramemter
> for crypttab would be processed, the other branch for fstab waits for the
> entry to appear. Since it has no idea what the backing device is it times
> out. 
> 
> Both seem to lack the concept of "wait forever".
> 
> So the workaround is to append "x-systemd.device-timeout=1234567890h" to all
> the mount options in fstab for devices backed by crypttab entries.


Yes, you need:

timeout=0 in crypttab for the crypt device and 
x-systemd-device-timeout=0 in fstab for the backing device
in order to suppress the timeouts.
Reason is that these are indeed 2 separate devices.
The crypt device is created by devicemapper on top of the backing device, but
for the system it is standalone.
That was the same, even before systemd. IMHO, the only difference is that
before, the backing device never timed out.

However, having a connection between the crypt and the backing device does make
sense.
Maybe it's worth a feature request?