Threads and the LDT (Intel-specific)?

[Scott Long <smlong@teleport.com>]

I'm trying to do something a bit unorthodox: I want to share the 
address space between threads, but I want a certain region of the 
address space to be writeable only for a particular thread -- for all 
other threads this region is read-only.

I've considered several approaches. I'll only go over the one I think 
is the best solution. I would appreciate any comments folks might have:

When linking the userspace program, I instruct the linker to avoid the 
region from VMA 0 to VMA 0x400000. Then, in the userspace code, I 
duplicate the GDT entries for CS, DS, ES, SS into the LDT, except that 
the new segments begin at 0x400000, instead of 0. I load the segment 
registers with these LDT entries. In the "write-allowed" thread I 
create an LDT entry to address VMA 0 through VMA 0x400000, in 
read-write mode. In all the other threads I create an LDT entry to 
address the same area in read-only mode. This gives me a 4M region at 
the bottom of memory that is accessible in different ways to different 
threads.

I can also use the LDT to point to thread-specific segments. IMHO this 
is much better than the stack trick used by linuxthreads. The problem 
is, I don't fully understand how to use modify_ldt(). Is anyone 
knowledgeable about this?

If anyone has any other suggestions, please let me know. If you are 
confused as to why I would ever want to do this in the first place, I'd 
be willing to go over it off the list.

Thanks,
Scott