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    SubjectRe: lockdep recursive locking detected (rcu_kthread / __cache_free)
    On Mon, Oct 03, 2011 at 03:46:11PM -0500, Christoph Lameter wrote:
    > On Mon, 3 Oct 2011, Paul E. McKenney wrote:
    >
    > > The first lock was acquired here in an RCU callback. The later lock that
    > > lockdep complained about appears to have been acquired from a recursive
    > > call to __cache_free(), with no help from RCU. This looks to me like
    > > one of the issues that arise from the slab allocator using itself to
    > > allocate slab metadata.
    >
    > Right. However, this is a false positive since the slab cache with
    > the metadata is different from the slab caches with the slab data. The slab
    > cache with the metadata does not use itself any metadata slab caches.

    Wouldn't it be possible to pass a new flag to the metadata slab caches
    upon creation so that their locks could be placed in a separate lock
    class? Just allocate a separate lock_class_key structure for each such
    lock in that case, and then use lockdep_set_class_and_name to associate
    that structure with the corresponding lock. I do this in kernel/rcutree.c
    in order to allow the rcu_node tree's locks to nest properly.

    Thanx, Paul


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