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    SubjectRe: [PATCH] slub: fix slab_pad_check()
    Christoph Lameter a écrit :
    > On Thu, 3 Sep 2009, Paul E. McKenney wrote:
    >
    >> 2. CPU 0 discovers that the slab cache can now be destroyed.
    >>
    >> It determines that there are no users, and has guaranteed
    >> that there will be no future users. So it knows that it
    >> can safely do kmem_cache_destroy().
    >>
    >> 3. In absence of rcu_barrier(), kmem_cache_destroy() would
    >> immediately tear down the slab data structures.
    >
    > Of course. This has been discussed before.
    >
    > You need to ensure that no objects are in use before destroying a slab. In
    > case of DESTROY_BY_RCU you must ensure that there are no potential
    > readers. So use a suitable rcu barrier or something else like a
    > synchronize_rcu...
    >
    >>> But going through the RCU period is pointless since no user of the cache
    >>> remains.
    >> Which is irrelevant. The outstanding RCU callback was posted by the
    >> slab cache itself, -not- by the user of the slab cache.
    >
    > There will be no rcu callbacks generated at kmem_cache_destroy with the
    > patch I posted.
    >
    >>> The dismantling does not need RCU since there are no operations on the
    >>> objects in progress. So simply switch DESTROY_BY_RCU off for close.
    >> Unless I am missing something, this patch re-introduces the bug that
    >> the rcu_barrier() was added to prevent. So, in absence of a better
    >> explanation of what I am missing:
    >
    > The "fix" was ill advised. Slab users must ensure that no objects are in
    > use before destroying a slab. Only the slab users know how the objects
    > are being used. The slab allocator itself cannot know how to ensure that
    > there are no pending references. Putting a rcu_barrier in there creates an
    > inconsistency in the operation of kmem_cache_destroy() and an expectation
    > of functionality that the function cannot provide.
    >



    Problem is not _objects_ Christoph, but _slabs_, and your patch is not working.

    Its true that when User calls kmem_cache_destroy(), all _objects_ were previously freed.
    This is mandatory, with or withou SLAB_DESTROY_BY_RCU thing

    Problem is that slub has some internal state, including some to-be-freed _slabs_,
    that User have no control at all on it.

    User cannot "know" slabs are freed, inuse, or whatever state in cache or call_rcu queues.

    Face it, SLAB_DESTROY_BY_RCU is internal affair (to slub/slab/... allocators)

    We absolutely need a rcu_barrier() somewhere, believe it or not. You can argue that it should
    be done *before*, but it gives no speedup, only potential bugs.

    Only case User should do its rcu_barrier() itself is if it knows some call_rcu() are pending
    and are delaying _objects_ freeing (typical !SLAB_DESTROY_RCU usage in RCU algos).

    I dont even understand why you care so much about kmem_cache_destroy(SLAB_DESTROY_BY_RCU),
    given that almost nobody use it. We took almost one month to find out what the bug was in first
    place...
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