On Sat, 2013-09-14 at 14:24 +0200, Manfred Spraul wrote:
> Hi Mike,
Hi,
> > Index: linux-2.6/ipc/sem.c
> > ===================================================================
> > --- linux-2.6.orig/ipc/sem.c
> > +++ linux-2.6/ipc/sem.c
> > @@ -247,11 +256,22 @@ static inline int sem_lock(struct sem_ar
> > */
> > lock_array:
> > spin_lock(&sma->sem_perm.lock);
> > + wait_array:
> > for (i = 0; i < sma->sem_nsems; i++) {
> > - struct sem *sem = sma->sem_base + i;
> > + sem = sma->sem_base + i;
> > +#ifdef CONFIG_PREEMPT_RT_BASE
> > + if (spin_is_locked(&sem->lock))
> > +#endif
> > spin_unlock_wait(&sem->lock);
> > }
> >
> I don't like this part of the change:
None of it is pretty, but the livelock is even less pretty ;-)
> It reads like a micro-optimization for spin_unlock_wait() within the
> ipc/sem.c code.
It's exactly that, hope to hammer fewer locks.
> If spin_unlock_wait() for CONFIG_PREEMPT_RT_BASE is broken, then the
> implementation of spin_unlock_wait() should be fixed.
But it's not broken, taking the lock lets PI see/fix inversion.
Preemptible locks are (necessary) evil incarnate.
-Mike
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