Dominic Sweetman wrote:
> > In any case, that's not the real problem. Linux user threads do not
> > have true separate stacks. They share their _entire_ address space;
> > the stacks are all bounded (default is 2MB) and grouped together at
> > the top of the available memory region.
> A comment by Kevin reminded me of the real constraint (which the
> experts probably take for granted): this system is supposed to work on
> shared-memory multiprocessors and multithreaded CPUs.
> In both cases two or more threads within an address space can be
> active simultaneously. On a multithreaded CPU (in particular) there's
> only one TLB, so memory (including any memory specially handled by the
> kernel) is all held in common. The *only* thing available to a user
> privilege program which distinguishes the threads is the CPU register
> (Well, and the stack, which is a difference inherited from the value
> in the stack pointer register. But the stack pointer is not really
> going to help much to return a thread-characteristic pointer or ID.)
Well, why not use the stack?
I am not quite familiar with the requirements on this "thread register",
but couldn't something like this be made to work:
#define TID *((sp & ~(STACK_SIZE-1)) + STACK_SIZE - TID_OFFSET)
It assumes a fixed maximum stack size (and alignment), which it should
be possible to meet (virtual memory is cheap). The STACK_SIZE could
probably even be a (process global!) variable if it is not desirable
to limit this at compile time.