Hi Dave,

Thanks a lot for your crystal clear explanation of my question.
Now I understood, actually I was thinking there will be only one stack
in kernel space which will be shared
by all apps.

Thanks again.


On Sat, Dec 10, 2011 at 1:00 PM, Dave Hylands [off-list ref] wrote:

Hi Subin,

On Sat, Dec 10, 2011 at 7:35 AM, subin gangadharan
[off-list ref] wrote:

quoted

Hi Guys,

I have a question related to task switching.Basically when user space
application switches to kernel (through sys calls),
which stack will be used for the user app.Mainly I want to know how
the current pointer implementation works.
I saw that in arm they are masking the sp with ~(THREAD_SIZE -1) and
getting the current thread info and from there its
taking the task pointer.

So here I am a little confused about the stack pointer since when it
switches the kernel it will be supervisory mode,so which
sp it's referencing.I know it will be a supervisory stack,but how this
stack will be related with the user app.

Every thread gets a kernel stack (typically of around 8K, although this can
vary by config options and architecture). This stack is created when the
thread is created. Whenever the code transitions from kernel space into user
space, the kernel sp is saved and the user sp is restored. When calling from
user-space into kernel space, the user-sp is saved and the kernel-sp is
restored.

Each thread has some thread-info which is stored on the low end of the
stack. Since the stacks are 8K is size and 8K aligned (or some other power
of 2 in size and the same power of 2 aligned), you can do a simple masking
operation with the sp to retrieve the thread-info.

--
Dave Hylands
Shuswap, BC, Canada
http://www.davehylands.com

-- 
With Regards
Subin Gangadharan

I am not afraid and I am also not afraid of being afraid.