How to release a stack with the context switch?

[cmpxchg64]

I now found out why my OS corrupts the stack in SMP mode:
The task_save_state() function stores context and then clears the task->is_running flag, which allows the use of the stack after a context switch in a parallel task_schedule().
BUT: The scheduler/interrupt handler is still running on top of the old context, which results in the same stack being used by two tasks. (only ring0->ring0 CS'es for now)

What strategy do you use to avoid this problem?
I cannot clear the task->is_running flag until the scheduler has finished execution: An idea would be to construct an artificial second context which switches to some function like context_switch(task* prev_task, context* real_ctx) using a stack on top of the new stack, which would clear the flag and then continue to the context of the task selected by the scheduler.
Are there other, possibly more simple ways to handle this?

Cheers cmpxchg64

[cmpxchg64]

After a litte thinking I now have found an even simpler solution, which should be pretty obvious: clear task->is_running and then perform the context switch without ever touching the stack again.
In practice this works by loading the esp of the new context into a register, then clearing the is_running flag and then doing the longjmp (which has to be implemented without a function call).

[onlyonemac]

It is generally considered bad form to change the topic subject halfway through a thread. However, if you want to highlight the fact that you've solved your problem, you can edit the first post and change the subject by adding "[SOLVED] " in front - this is the standard way to mark a thread as solved, so that firstly you won't keep receiving replies from people who don't read the whole thread before replying and secondly others can find your solution more easily if they are having the same problem.

[Combuster]

What strategy do you use to avoid this problem?

The scheduler has its own per-core stack. That way I also know exactly where a suspended process' registers are without having to know what the scheduler stack does.