Hi,
I was wondering about few things concerning the function copy_thread(...) in
arch/.../kernel/process.c. Here is the code for the m68k architecture (no
mmu), which I try to edit:
int copy_thread(int nr, unsigned long clone_flags, unsigned long usp,
unsigned long topstk, struct task_struct * p, struct pt_regs * regs)
{
1 struct pt_regs * childregs;
2 struct switch_stack * childstack, *stack;
3 unsigned long stack_offset, *retp;
4
5 stack_offset = KTHREAD_SIZE - sizeof(struct pt_regs);
6 childregs = (struct pt_regs *) ((unsigned long) p + stack_offset);
7
8 *childregs = *regs;
9 childregs->d0 = 0;
10
11 retp = ((unsigned long *) regs);
12 stack = ((struct switch_stack *) retp) - 1;
13
14 childstack = ((struct switch_stack *) childregs) - 1;
15 *childstack = *stack;
16 childstack->retpc = (unsigned long)ret_from_fork;
17
18 p->thread.usp = usp;
19 p->thread.ksp = (unsigned long)childstack;
20 /*
21 * Must save the current SFC/DFC value, NOT the value when
22 * the parent was last descheduled - RGH 10-08-96
23 */
24 p->thread.fs = get_fs().seg;
return 0;
}
Motorola defines "struct switch_stack" as some kind of extended stack, since
they define some registers like d6, d7, a3, a4, retpc etc. which are not
defined in pt_regs structure. But that doesn't matter since I don't wan't to
use switch_stack at all (I decided to declare all registers in pt_regs). But
these are the things I don't understand:
1) In lines 5 and 6 they do some pointer arithmetic in order to calculate
the bottom of the childstack. Am I right? I guess KTHREAD_SIZE is the size
of the kernel stack and is 2K, so they get a stack_offset. Why do they need
stack_offset?
2) After line 6 has been executed, the variable childregs points to p +
stack_offset. Is p the top of the previous stack? If so, then do they
overwrite the previous stack? I mean, doesn't the stack grow downwards in
linux? p + stack_offset is greater the p and than the previous stack would
be overwritten.
Best regards,
Paul
--
5 GB Mailbox, 50 FreeSMS http://www.gmx.net/de/go/promail
+++ GMX - die erste Adresse für Mail, Message, More +++
--
Kernelnewbies: Help each other learn about the Linux kernel.
Archive: http://mail.nl.linux.org/kernelnewbies/
FAQ: http://kernelnewbies.org/faq/
