Re: [9fans] Re: Threads: Sewing badges of honor onto a Kernel

[Linus Torvalds <torvalds@osdl.org>]

On Fri, 27 Feb 2004, Lucio De Re wrote:
> >
> > You literally need _one_ operation: you need the operation of "give me the
> > TLS pointer" (well, your thread setup code obviously needs a way to set
> > the pointer when creating a thread too).
> >
> Yes, but...
>
> Unless I'm missing the wood for the trees, this returns a pointer.
> Where do I put _it_ so no other thread, choosing to do likewise,
> decides to stomp all over it?

Your register state? Or you save it to the (shared) VM, by using another
private pointer (ie the stack pointer).

It all boils down to one thing: the _only_ state that is always truly
private in your thread is your register state. But exactly because the
register state is private, you now have ways of turning the _rest_ of your
state (that isn't private) into your own private areas.

So while the VM is shared across all different threads, all threads
obviously have their own stack pointer registers, and they'd better be
pointing to different _parts_ of that shared VM, or your threads won't
work at all.

	"Any problem in computer science can be solved with another layer
	 of indirection."
			David Wheeler

In other words, your registers fundamentally _are_ your private space, and
everything else follows from that.

So your stack is "private"  in the sense that nobody else uses it unless
you explicitly give a pointer to it (modulo bugs, of course ;).

			Linus