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SubjectRe: [patch 2/4] lglock: introduce special lglock and brlock spin locks
On Fri, Jun 04, 2010 at 08:03:27AM -0700, Paul E. McKenney wrote:
> On Fri, Jun 04, 2010 at 04:43:09PM +1000, Nick Piggin wrote:
> > This patch introduces "local-global" locks (lglocks). These can be used to:
> >
> > - Provide fast exclusive access to per-CPU data, with exclusive access to
> > another CPU's data allowed but possibly subject to contention, and to provide
> > very slow exclusive access to all per-CPU data.
> > - Or to provide very fast and scalable read serialisation, and to provide
> > very slow exclusive serialisation of data (not necessarily per-CPU data).
> >
> > Brlocks are also implemented as a short-hand notation for the latter use
> > case.
> >
> > Thanks to Paul for local/global naming convention.
>
> ;-)
>
> One set of questions about how this relates to real-time below.
>
> (And I agree with Eric's point about for_each_possible_cpu(), FWIW.)

...

> > + void name##_lock_init(void) { \
> > + int i; \
> > + LOCKDEP_INIT_MAP(&name##_lock_dep_map, #name, &name##_lock_key, 0); \
> > + for_each_possible_cpu(i) { \
> > + arch_spinlock_t *lock; \
> > + lock = &per_cpu(name##_lock, i); \
> > + *lock = (arch_spinlock_t)__ARCH_SPIN_LOCK_UNLOCKED; \
> > + } \
> > + } \
> > + EXPORT_SYMBOL(name##_lock_init); \
> > + \
> > + void name##_local_lock(void) { \
> > + arch_spinlock_t *lock; \
> > + preempt_disable(); \
>
> In a -rt kernel, I believe we would not want the above preempt_disable().
> Of course, in this case the arch_spin_lock() would need to become
> spin_lock() or some such.
>
> The main point of this approach is to avoid cross-CPU holding of these
> locks, correct? And then the point of arch_spin_lock() is to avoid the
> redundant preempt_disable(), right?

Yes. Preempt count and possibly lockdep will have issues with taking
so many nested locks in the write path.

The brlock version of this does avoid holding cross-CPU locks in the
fastpath. The lglock version used by files_list locking in the next
patch does need to sometimes take a cross-CPU lock.



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