Re: lockdep complaints in slab allocator

[Peter Zijlstra]

On Fri, 2009-11-20 at 06:48 -0800, Paul E. McKenney wrote:
> On Fri, Nov 20, 2009 at 01:05:58PM +0200, Pekka Enberg wrote:
> > Peter Zijlstra kirjoitti:
> >> On Fri, 2009-11-20 at 12:38 +0200, Pekka Enberg wrote:
> >>>
> >>> On Fri, Nov 20, 2009 at 11:25 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 
> >>> wrote:
> >>>>  2) propagate the nesting information and user spin_lock_nested(), given
> >>>> that slab is already a rat's nest, this won't make it any less obvious.
> >>> spin_lock_nested() doesn't really help us here because there's a
> >>> _real_ possibility of a recursive spin lock here, right? 
> >> Well, I was working under the assumption that your analysis of it being
> >> a false positive was right ;-)
> >> I briefly tried to verify that, but got lost and gave up, at which point
> >> I started looking for ways to annotate.
> >
> > Uh, ok, so apparently I was right after all. There's a comment in 
> > free_block() above the slab_destroy() call that refers to the comment above 
> > alloc_slabmgmt() function definition which explains it all.
> >
> > Long story short: ->slab_cachep never points to the same kmalloc cache 
> > we're allocating or freeing from. Where do we need to put the 
> > spin_lock_nested() annotation? Would it be enough to just use it in 
> > cache_free_alien() for alien->lock or do we need it in cache_flusharray() 
> > as well?
> 
> Hmmm...  If the nc->lock spinlocks are always from different slabs
> (as alloc_slabmgmt()'s block comment claims), why not just give each
> array_cache structure's lock its own struct lock_class_key?  They
> are zero size unless you have lockdep enabled.

Because more classes:

 - takes more (static/limited) lockdep resources

 - make more chains, weakening lock dependency tracking
   because it can no longer use the state observed in one branch
   on state observed in another branch.

Suppose you have 3 locks and 2 classes, lock 1 and 2 part of class A and
lock 3 of class B

Then if we observe 1 -> 3, and 3 -> 2, we'd see A->B and B->A, and go
yell. Now if we split class A into two classes and these locks get into
separate classes we loose that cycle.

Now in this case we want to break a cycle, so the above will be correct,
but all resulting chains will be equivalent for 99% (with the one
exception of this funny recursion case) wasting lots of resources and
state matching opportunity.

Therefore it would be much better to use the _nested annotation if
possible.

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