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    SubjectRe: lockdep complaints in slab allocator
    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 <>
    > >>> 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

     \ /
      Last update: 2009-11-20 16:21    [W:0.022 / U:161.800 seconds]
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