HAL has broken handling of the temporary devices that cryptsetup creates when opening a LUKS volume. Documented at the bottom of the linked page. (Note: I am not the author of said page). Attaching patch to HAL and modified ebuilds for 5.9.1-r1 and 5.10 using said patch.
Created attachment 144719 [details, diff] Patch to HAL fixing handling of temporary cryptsetup devices
Created attachment 144721 [details] Ebuild for sys-apps/hal-0.5.9.1-r1 using above patch
Created attachment 144723 [details] Ebuild for sys-apps/hal-0.5.10 using above patch
According to this: http://bugs.freedesktop.org/show_bug.cgi?id=12899#c2 this is actually a bug in udev that has been fixed in commit a0092d28.
is it still a problem with >=hal-0.5.11 and >=udev-117 ?
Well, commit 0f2dfc66d1f92aaf464adadeaf327d5a31e0ebcb in HAL's git repo fixed it for me, so if this commit is before that version was tagged, then the bug is no longer present.
I am not sure about udev but, anyway, hal-0.5.11 shouldn't be affected by this as it includes workaround in http://bugs.freedesktop.org/show_bug.cgi?id=12899#c3 I think that this can be closed (I can do it if you let me ;-))
according to git history the fix should be in 0.5.11 so closing this fixed.