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    SubjectRe: SLUB defrag pull request?
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    On Tue, 21 Oct 2008, Christoph Lameter wrote:
    > The only way that a secure reference can be established is if the
    > slab page is locked. That requires a spinlock. The slab allocator
    > calls the get() functions while the slab lock guarantees object
    > existence. Then locks are dropped and reclaim actions can start with
    > the guarantee that the slab object will not suddenly vanish.

    Yes, you've made up your mind, that you want to do it this way. But
    it's the _wrong_ way, this "want to get a secure reference for use
    later" leads to madness when applied to dentries or inodes. Try for a
    minute to think outside this template.

    For example dcache_lock will protect against dentries moving to/from
    d_lru. So you can do this:

    take dcache_lock
    check if d_lru is non-empty
    take sb->s_umount
    free dentry
    release sb->s_umount
    release dcache_lock

    Yeah, locking will be more complicated in reality. Still, much less
    complicated than trying to do the same across two separate phases.

    Why can't something like that work?

    Miklos


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