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    SubjectRe: [PATCH 22/25] Generic dynamic per cpu refcounting
    On Thu, Nov 29, 2012 at 07:59:53PM +0100, Andi Kleen wrote:
    > On Thu, Nov 29, 2012 at 10:57:20AM -0800, Kent Overstreet wrote:
    > > On Thu, Nov 29, 2012 at 10:45:04AM -0800, Andi Kleen wrote:
    > > > Kent Overstreet <koverstreet@google.com> writes:
    > > >
    > > > > This implements a refcount with similar semantics to
    > > > > atomic_get()/atomic_dec_and_test(), that starts out as just an atomic_t
    > > > > but dynamically switches to per cpu refcounting when the rate of
    > > > > gets/puts becomes too high.
    > > >
    > > > This will only work if you put on the same CPU as you get, right?
    > >
    > > Nope, no such restriction.
    >
    > I don't see how you ensure you're doing the __this_cpu_dec on the same
    > CPU as you did the get

    I'm not.

    (I probably should've documented this a bit more before I sent it
    out...)

    The trick is that we don't watch for the refcount hitting 0 until we're
    shutting down - so this only works if you keep track of your initial
    refcount. As long as we're not shutting down, we know the refcount can't
    hit 0 because we haven't released the initial refcount.

    When we do want to shutdown, the user calls percpu_ref_kill() which
    converts the percpu ref back to a single atomic ref, calls
    synchronize_rcu(), then sets the ref's state to PCPU_REF_DEAD.

    Only then does the caller drop the initial ref, and percpu_ref_put()
    only does atomic_dec_and_test() when the ref is dead - otherwise it's
    just doing a decrement.

    Also, with the percpu refs - you can have all your gets happening on one
    cpu, and all your puts happening on another - the percpu refs are
    unsigned ints so overflow isn't undefined, and if they wrap they'll
    still sum to the right value when we go to shut things down in
    percpu_ref_kill().


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