Hello Anna,
Thanks for the reply. Maybe I'v imagined an impractical scenario:
1. A yield to B. B->sc->scYieldFrom=A
2. B run then block.
3. A run then yield to B again.
4. Now at the beginning, setConsumed() called; because B is not runnable, so return_now=true and setConsumed() called again at the end.
For cooperative threads, this indeed shouldn't happen but it depends. For example if B can block, then great care should be taken to forbid this edge case.
Regards,
laokz
------------------ Original ------------------
From: "Anna Lyons";
Date: Mon, Apr 6, 2020 12:16 PM
To: "laokz";"devel";
Subject: Re: [seL4] setConsumed() in seL4_SchedContext_YieldTo
P {margin-top:0;margin-bottom:0;} Hi,
Thanks for the observation! It's true that setConsumed should only be called once per kernel entry, and it's not obvious just from the code snippets to observe that this is true.
However, the semantics of YieldTo mean that both of these code paths cannot be executed in the same kernel entry.
If return_now is true, this means that the current thread will be switched to and sc->scYieldFrom should be NULL (as the YieldTo call has completed). This is usually the case if the calling thread YieldTo a lower priority thread, or a thread that is not runnable. On the other hand if return_now is false, the YieldTo resulted in the thread being YieldedTo being switched to, so we return the result of the YieldTo on a different kernel entry (when we run the thread that called YieldTo again).
With this context, take a look at the if block here: https://github.com/seL4/seL4/blob/master/src/object/schedcontext.c#L191
Where you can see that we only set sc->scYieldFrom when we also set return_now to false.
Glad to see someone looking at the MCS kernel.
Anna.
