Re: current pointer question/suggestion
Alan Cox (alan@lxorguk.ukuu.org.uk)
Fri, 17 Jul 1998 21:54:50 +0100 (BST)
>
> On Fri, Jul 17, 1998 at 01:52:37PM +0100, Alan Cox wrote:
> >
> > The wait queue is a linked list of who is waiting. That list spans the
> > kernel stacks of each process. When you walk a wakeup list you walk
> > across all those stacks.
> >
>
> Ok - I understand. My point was not to put the _stack_ at a constant
> address, but to put _current_ on a constant address by having one
> "kernel" page mapped differently in each task.
>
> Currently, the task_struct and the stack are allocated together to
> make access to current fast. With current at a constant address,
> there would be no need to have the stack in the same area and if we
> can do with a 4k stack, no need to allocate a 8k region. But the main
> point was that it is slightly faster to do "current = const" than
> "current = %esp & ~8191" which is slightly faster than "current =
> *(%esp & ~4095)" [is 2.0 something like that?].
>
> The task_struct for current would be mapped two places in memory so
> the problem would be to make sure that we don't put any pointers to
> current into kernel datastructures but always use a pointer (I called
> it current->this) which points to the linear address for the
> task_struct.
>
> If this is feasible, it can be used for other structures as well, but
> at that time things probably get a bit hairy and there must be a set
> of macros that are used instead of the '&' operator when the address
> should be accessed outside of the current process.
>
> There could also be a problem if other parts of the kernel wanted to
> poke into arbitrary parts of the stack of other processes since such a
> stack could have a pointer to current which wouldn't have any meaning
> to the poking process. [And based on the comments I got, I thought
> some part of the kernel did just that and I couldn't understand why!].
>
> astor
>
> --
> Alexander Kjeldaas, Guardian Networks AS, Trondheim, Norway
> http://www.guardian.no/
>
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