Aw: Re: How to switch thread stack between threads?

[Uwe]

> Gesendet: Donnerstag, 15. Juli 2021 um 21:21 Uhr
> Von: "Alexander Tormasov via users" <users at lists.genode.org>
> An: "Genode users mailing list" <users at lists.genode.org>
> Cc: "Alexander Tormasov" <a.tormasov at innopolis.ru>
> Betreff: Re: How to switch thread stack between threads?
>
> >> 
> >> So, to fix it I need during switch of context to non-local thread (setcontext() or even longjump() functions) I should update these data to current running stack.
> > user level switching is only valid within the same thread. The only way to do this is to do a local
> > user level switch to a user level thread that immediately blocks the os level thread and wakes the os level
> > thread, that is blocked in the same procedure and corresponds to the target user level thread. At wakeup that
> > user level thread, which was blocked at the os level, reads the target user level thread and makes a local
> > user level switch to it.
> > The Mutex on which the user level threads blocks (at least its address) needs to be part of the context.
> 
> probably I give a wrong picture of operations
I did understand you the first time around. But I have to disappoint you.
What you want is IMPOSSIBLE! At least in genode. Because you can't create such a context.
> 1. I run arbitrary function with stack associated with first thread
Every stack implies (is bound to) an os level thread.
> 2. I copy current context using getcontext and store it somewhere
You must store a pointer to an os level object that holds the os level thread (mutex is fine)
with the context to be able to later resume in the correct context.
> 3. I stop doing function from 1, by switching to another function/stack associated with thread 1
That is possible.
> 4. after some time I create a new os thread and run some code inside it
> 5. then inside 2 thread I take old context from 2 above and perform setcontext from inside 2 thread to replace current function with state in the thread to the first one
You can not use context from 2 in another thread. Alternatively you can reconstruct the call chain from the first thread in the second thread with Duffs Device (https://en.wikipedia.org/wiki/Duff's_device#See_also)
And then you can construct a mirror context, which is the first context but in the second thread.
> 
> So, I don’t need mutexes and wait for something - I have a time gap between suspend of function in os 1 and it continuation on thread 2.
The mutexes have another purpose. They regulate the os level threads when user level threads yield.
> during continuation I already switched to thread 2… 
> just need to associate old stack with it (it contains state of function/stack from 1)
> 
> 
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