Re: per-thread global variables

["J.A. Magallon" <jamagallon@able.es>]

On 2002.07.13 Alan Cox wrote:
>On Sat, 2002-07-13 at 01:12, J.A. Magallon wrote:
>> On IRIX, you have a
>> 
>> // Fixed address space always private for each process/thread
>> #define PRDA        ((struct prda *)0x00200000L)
>> 
>> struct prda*	thrprda;
>> int*	self;
>> 
>
>Which goes to show they weren't planning for clone like performance. A
>single unshared page means two seperate mm structs, two sets of page
>tables and switches causing TLB flushes. All because
>
>	movl %esp, %eax
>	andl $ffff8000, %eax
>	
>and casting that is 'hard'
>

OK, I do not know a word on x86 assembler, but lets see if at least I understand
this.

stack = reserve 32K+sizeof(my_private_area) (say, 1Kb)
clone() lying about stack start: stack + 32K - 1
   (private on top and stack grows downwards)

   |---- stack ----||--- private -----|
   0                32K               33k
                   ^
                   stack passed to clone()

void* getpriv()
{
  // get my stack frame pointer, can be anywhere depending on my
  // 
  movl %esp, %eax
  |---- stack ----||--- private -----|
           ^
           %eax
  // Round it to 32k multiple, cause I know the thread stack is 32k aligned
  // So I have the thread stack end, ie, bottom
  andl $ffff8000, %eax
  |---- stack ----||--- private -----|
  ^
  %eax
  // Move after top of stack
  addl 32K,%eax
  |---- stack ----||--- private -----|
                   ^
                   %eax
  // and I have the begining of my private area, upwards
}

Could it even be correct ?

TIA

-- 
J.A. Magallon             \   Software is like sex: It's better when it's free
mailto:jamagallon@able.es  \                    -- Linus Torvalds, FSF T-shirt
Linux werewolf 2.4.19-rc1-jam3, Mandrake Linux 8.3 (Cooker) for i586
gcc (GCC) 3.1.1 (Mandrake Linux 8.3 3.1.1-0.7mdk)