> I'd like to give it a shot (implementing pthreads support for the Hurd > I mean).
That would be great! I would very much like this to be as OS-independent as possible, and to clean up and generalize the linuxthreads code as you see fit so that we have a single clean implementation. If doing this right turns out to necessitate changing the libpthread ABI for linux, we'll find a way to deal. > It seems that on the i386, it is possible to use the LDT to > store some thread-specific data. Mach supports a per-thread LDT and > even has an RPC interface to manipulate it. You could do that, but I think it is simpler to start with the cthreads approach of using fix-sized aligned stacks for threads and storing data at the base of the stack. This doesn't implement the stackaddr pthread attribute, but you could probably get it going faster. I have no particular confidence in the ldt code in gnumach, since it has not been well exercised in a very long time. > I have to admit that I stole this idea from Ulrich (there is a file in > the linuxthreads add-on that contains some code, but it isn't actually > used). Nevertheless, I'll try to make things as machine/OS independent > as possible. It's not a new idea, and there are other variations using the x86 segment registers besides LDT. On most other current machines (RISC), it is canonical to dedicate a register for this. As long as you isolate that magic in a sysdeps file, we can change it later and for different kernel setups. (Another implementation that can be more efficient on the x86 is to have a segment register point to a dedicated word in high memory that the kernel changes on context switches. That avoids changing segmentation state on the chip for every thread switch.) I think Thomas objects to the LDT stuff because it's machine-specific hairy (and crufty old code in mach), and I sympathize with his conservatism. But truly sufficient pthreads support requires supporting the stackaddr and stacksize pthread attributes, and then you cannot make universal assumptions about stack layout. It's just the way it is.
