Re: ci_mtx_count issue on NetBSD/Milkymist

[Yann Sionneau <yann.sionneau%gmail.com@localhost>]

Le 25/02/14 22:13, Mindaugas Rasiukevicius a écrit :
> Yann Sionneau <yann.sionneau%gmail.com@localhost> wrote:
> > > It is a reference count on the IPL raised by spin-mutexes.  The IPL
> > > should be lowered (straight to IPL_NONE) on a last reference.  Notice
> > > that this is a negative counter - mutex_spin_enter() decrements, while
> > > mutex_spin_exit() incremenets.  Not the other way round.
> > Why should curcpu()->ci_mtx_count be -1 during mi_switch()?
> > Using watchpoint I discovered that I never access cpu0->ci_mtx_count.
> It asserts for -1 because only the LWP lock should be held at that point.
Ok, but locking the LWP lock won't make ci_mtx_count be -1 because the 
lock is a MUTEX_DEFAULT and not a MUTEX_SPIN.
> > This, as you said, should be incremented/decremented upon
> > mutex_spin_enter or exit, but in my case mutex_spin_enter() is aliased
> > to mutex_vector_enter because I don't have any stubs and anyway I have
> > enabled LOCKDEBUG kernel option.
> If you have __HAVE_SIMPLE_MUTEXES defined and __HAVE_{SPIN_}MUTEX_STUBS
> undefined, then MD code does not need any extra handling.
Ok, good :)
> > mutex_spin_enter() only touches curcpu()->ci_mtx_count when the mutex is
> > a MUTEX_SPIN, I put a breakpoint in mutex_init() and tested all "type"
> > argument: I never initialize any MUTEX_SPIN.
> > It seems I only use MUTEX_DEFAULT or MUTEX_DRIVER.
> You should generally use only MUTEX_DEFAULT, mutex_init(9) will determine
> the type from IPL value.
ok
> > Do I miss a MUTEX_SPIN somewhere? which spin mutex is supposed to be
> > held before calling mi_switch()? (or at least before the KASSERTMSG
> > happens).
> That assert is generally for catching locking bugs.  Perhaps you missed
> mutex_exit()/mutex_spin_exit() somewhere in your MD code?  Get a backtrace
> and inspect the code?
Well, it's 0 because no spin lock is locked. So I don't think I missed a 
mutex_spin_exit() ... I think I don't understand the situation at all 
here :/

Thanks for your help anyway :)

Cheers,

--
Yann Sionneau