task switching and current pointer implemenation question

[Dave Hylands]

Hi Subin,

On Sat, Dec 10, 2011 at 7:35 AM, subin gangadharan <
subingangadharan at gmail.com> wrote:

> 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
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