Kernel Stack vs User Mode Stack Post: 302586912

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Top Forums UNIX for Dummies Questions & Answers Kernel Stack vs User Mode Stack Post 302586912 by ygemici on Tuesday 3rd of January 2012 03:54:52 PM

01-03-2012

Registered User

Quote:

Originally Posted by saurabhkoar

@fpmurphy,

That means in Linux each process has a user mode stack and a corresponding Kernel mode stack? So, there is two stacks per process?

actually they are used on the separate [cpu processing] modes.
and in this state lets we split to as modes(user/kernel) for better understanding of its.
shortly,linux process can operates in two modes:

* in user_mode
any process which runs the in user mode refers to private(own) stack area that is used for holds local variables,parameters,frame pointer(very useful while detect relative addres of any func local variables after pushes,pops especially in debug processing[kernel oops]),temporary(as a cpy) values(like in c,c++,java),return address from funcs and automatic variables within functions.

* in kernel mode (after switching to kernel mode by [switch_to] ), portion of the process (code areas),its addresses and core kernel data, starts to use the kernel stack.
[as additonal infos; hardware context informations(like contents of all registers in user mode) also are saved in the kernel stack while the hardware context processing]

linux kernel has many complex structures like kernel control path that is sequence of instructions(* executed several kernel functions) executed by a kernel for an interrupt,system call or exception handling.
so these instructions are belonging to different processes.therefore each kernel path must refers to its kernel stack(8k)
[actually 8140 bytes 52 byte save for thread_info structure]
you can define the size of kernel stacks while compaling if you see the like this message

Code:

Use 4Kb for kernel stacks instead of 8Kb (4KSTACKS) [N/y/?] n

and user mode process can not know where is the address of own kernel stack memory area; (pointer) address, therefore cannot accessed to kernel stack area in the user mode.

while switching to kernel mode with mode switch,process will continue to work in the reentrance kernels.
so it is a reentrance kernel! user mode process executions can be resume in kernel mode and continues processing for new processes by kernel schedulers.[assisgns the another Cpu to the process with schedule()]
but on a single processor system,one process will be process in the Cpu and new processes blocked until the current process turn.

I would recommend for more information below usefull links..
Understanding the Linux Kernel, 3rd Edition
4K Stacks in 2.6 [LWN.net]

regards
ygemici

ygemici

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LEARN ABOUT SUSE

sigreturn

SIGRETURN(2)						     Linux Programmer's Manual						      SIGRETURN(2)

NAME

       sigreturn - return from signal handler and cleanup stack frame

SYNOPSIS

       int sigreturn(unsigned long __unused);

DESCRIPTION

       When  the  Linux  kernel  creates the stack frame for a signal handler, a call to sigreturn() is inserted into the stack frame so that upon
       return from the signal handler, sigreturn() will be called.

       This sigreturn() call undoes everything that was done--changing the process's signal mask, switching stacks (see sigaltstack(2))--in  order
       to  invoke  the signal handler: it restores the process's signal mask, switches stacks, and restores the process's context (registers, pro-
       cessor flags), so that the process directly resumes execution at the point where it was interrupted by the signal.

RETURN VALUE

       sigreturn() never returns.

FILES

       /usr/src/linux/arch/i386/kernel/signal.c
       /usr/src/linux/arch/alpha/kernel/entry.S

CONFORMING TO

       sigreturn() is specific to Linux and should not be used in programs intended to be portable.

NOTES

       The sigreturn() call is used by the kernel to implement signal handlers.  It should never be called directly.  Better yet, the specific use
       of the __unused argument varies depending on the architecture.

SEE ALSO

       kill(2), sigaltstack(2), signal(2), signal(7)

COLOPHON

       This  page is part of release 3.25 of the Linux man-pages project.  A description of the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2008-06-26							      SIGRETURN(2)