When updating the kernel, pacman also removes the currently running kernel, including the modules.
This may prevent the user from mounting new filesystems until a reboot. For example iso9660 or udf (for dvds).
When I encountered this problem I thought it was a vlc bug or something until I manually tried to mount the disc.
So it may be only a minor issue, but it can cause a lot of confusion.
Yes, this is the default pacman behaviour and known to cause problems when a kernel module is requested before reboot. Work around: extract the current kernel's package somewhere in /tmp and insmod the module manually.
Sorry for the late reply.
How can we fix it? Rebooting is in my case easier than manually insmoding the necessary modules.
But I don't want it to stay this way. It is just unnecessary pain. And we are using free software. So it should be fixable and fixed.
If it's a known problem, others should already have come up with ideas. Are you aware of any of these?
If someone here is interested, I found this solution on reddit (https://www.reddit.com/r/archlinux/comments/4zrsc3/keep_your_system_fully_functional_after_a_kernel/).
It is not perfect, though. Read the comments for potential problems.
you should reboot your pc after every kernel update, it is better... if yu can not reoot, better if you wait with upgrade your OS ...
It would be best if Arch supported live kernel update/patching-
The thing is I'm using a cron job to update.
Yes, I know that it's not recommended. But I don't want to always do it manually. A system should be able to update itself automatically. On pacman that is not (officially) supported, and I hate that.