"Tommy S. Christensen" <email@example.com> wrote:
> 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.
> > Quite.
> > 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
> > set.
> > (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.
Thanks for writing this up. I had the same thought over dinner,
but I'm throughly discredited today, and it's better that it came
from someone else. ;-)