Gilles Chanteperdrix wrote:
> Jan Kiszka wrote:
>  > Gilles Chanteperdrix wrote:
>  > > Jan Kiszka wrote:
>  > >  > As the #ifdef forest was cut down, I once again looked at xnlock_put.
>  > >  > Why do you have this safety check for the owner also in production 
> code?
>  > > 
>  > > Because only one broken xnlock_put could entail a chain reaction of
>  > > broken xnlock sections with code on multiple CPU violating critical
>  > > sections. With the test, we prevent the chain reaction. But I agree this
>  > > check should not be silent.
>  > 
>  > When there is a bug, then there is bug and we are hosed. That's why we
>  > have debug checks for finding such cases in advance. Here I was talking
>  > about such a debug check in a hot path on a _production_ system, and
>  > that check even had no fault recovery. That appeared pointless to me.
>  > 
>  > Just to avoid misunderstandings: This version is not different from the
>  > old one if XENO_OPT_DEBUG_NUCLEUS is on, the switch which was introduced
>  > to cover specifically lock debugging.
>  > 
>  > Do you have an idea for some cheap fault recovery for broken locking
>  > that we should put in instead?
> 
> The fault recovery is to leave the xnlock untouched, in order to avoid
> the chain reaction I was talking about.

But you may later deadlock on it when trying to reacquire this
unbalanced lock. It can help against double releases, granted. Still,
such cases should be eliminated _ahead_ of release via review, so that
one can finally go for the fast unchecked version in the matured system.

> Anyway, I think even production
> code should contain some sanity checks, and this one looks cheap.

I'm fine with a simple check as well. But there should be an _option_
for such checks, even more if they are supposed to be inlined.
Uninlining reduces this pressure, but it still adds text size to the hot
path.

Jan

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