Re: linux-next: manual merge of the tip tree

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Oct 17, 2013 at 12:28:59PM +1100, NeilBrown wrote:
> I always run with lockdep enabled, and I have done at least basic testing

Very good!

> > 
> > Stuff like:
> > 
> > +	for (i = 0; i < NR_STRIPE_HASH_LOCKS; i++)
> > +		spin_lock_init(conf->hash_locks + i);
> > 
> > And:
> > 
> > +static void __lock_all_hash_locks(struct r5conf *conf)
> > +{
> > +	int i;
> > +	for (i = 0; i < NR_STRIPE_HASH_LOCKS; i++)
> > +		spin_lock(conf->hash_locks + i);
> > +}
> > 
> > Tends to complain real loud.
> 
> Why is that?
> Because "conf->hash_locks + i" gets used as the "name" of the lockdep map for
> each one, and when they are all locked it looks like nested locking??

Exactly so; they all share the same class (and name) because they have
the same init site; so indeed the multiple lock will look like a nested
lock.

> Do you have a suggestion for how to make this work?
> Would
>     spin_lock_nested(conf->hash_locks + i, i)
> do the trick?

	spin_lock_nest_lock(conf->hash_locks + i, &conf->device_lock);

Would be the better option; your suggestion might just work because
NR_STRIP_HASH_LOCKS is 8 and we have exactly 8 subclasses available, but
any increase to NR_STRIPE_HASH_LOCKS will make things explode again.

The spin_lock_nest_lock() annotation tells that the lock order is
irrelevant because all such multiple acquisitions are serialized under
the other lock.

Also, if in future you feel the need to increase NR_STRIP_HASH_LOCKS,
please keep it <= 64 or so; if you have a need to go above that, please
yell and we'll see if we can do something smarter.

This is because of:
 - each spin_lock() increases preempt_count and that's 8 bits; we
   wouldn't want to overflow that
 - each consecutive nested spin_lock() increases the total acquisition
   wait-time for all locks. Note that the worst case acquisition time
   for even a single hash lock is gated by the complete acquisition time
   of all of them in this scenario.