about platform_execute in usb_drv

[jaeyong yoo]

It enlightened me. Thanks a lot!

Jaeyong

On Wed, Jan 9, 2013 at 7:46 PM, Norman Feske
<norman.feske at ...1...>wrote:

> Hello Jaeyong,
>
> >> This function is used to switch the stack and execute a given function.
> >
> > Why do we have to switch the stack?
> > For instance, what would happen if we do not switch the stack?
>
> in contrast to normal function calls, user-level threads are executed
> without the need to return from a function. For example, let us assume
> that a user-level thread executes the following code:
>
>   void thread_entry()
>   {
>     int signal_count = 0;
>     for (;;) {
>       wait_for_signal();
>       signal_count++;
>       printf("signal_count is %d", signal_count);
>       ...
>     }
>   }
>
> Here, the variable 'signal_count' is allocated on the thread's stack
> because it is a local variable. The function 'wait_for_signal()' blocks
> the user-level thread for an incoming event and invokes scheduler of
> other user-level threads. So here, a user-level context switch might
> happen.
>
> Now, imagine that you have two of such threads running. Let's call them
> A and B. If they would share one stack, and the threads were scheduled
> in the order A, then B, the resulting stack would look as follows:
>
>   ------------------------------------------ <- stack top
>    context of B (i.e., 'signal_count' of B)
>   ------------------------------------------
>    context of A (i.e., 'signal_count' of A)
>   ------------------------------------------
>
> How would the stack look after the user-level scheduler would now pick
> A? If we just switched to the last stack pointer used by A, the picture
> would look like this:
>
>   ------------------------------------------
>    context of B (i.e., 'signal_count' of B)
>   ------------------------------------------ <- stack top
>    context of A (i.e., 'signal_count' of A)
>   ------------------------------------------
>
> Now, thread A executes the 'printf' function, which uses the stack and
> thereby overwrites the context of 'B'. After returning from 'printf',
> the stack would look like this:
>
>   ------------------------------------------
>    left-overs of the printf function
>   ------------------------------------------ <- stack top
>    context of A (i.e., 'signal_count' of A)
>   ------------------------------------------
>
> The context of B has been overwritten by the function call frames used
> by the 'printf' function and its subroutines.
>
> By hosting the stack of each user-level thread in a distinct memory
> area, such situations cannot happen. The threads can be executed in any
> order without overwriting each other's context.
>
> Cheers
> Norman
>
> --
> Dr.-Ing. Norman Feske
> Genode Labs
>
> http://www.genode-labs.com · http://genode.org
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