Well, Alan did say "user mode stuff".
The i386 is indeed very very broken when it comes to read-only pages in
kernel space. The kernel tries to handle this, but the i386 braindamage
does result in threads being inherently unsafe because there are various
nasty race conditions that a thread library could hit.
This is fixed in the i486 and up, and the threads race problem only
shows up for code that tries to be malicious on purpose, so it is mostly
harmless (hint: if you're using a 386 as a server that accepts logins,
you should probably try to hit somebody for a 486 - there must be tons
of them lying around).
Linus