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    SubjectRe: rcu-refcount stacker performance
    On Thu, Jul 14, 2005 at 08:44:50AM -0500, serue@us.ibm.com wrote:
    > Quoting Paul E. McKenney (paulmck@us.ibm.com):
    > > My guess is that the reference count is indeed costing you quite a
    > > bit. I glance quickly at the patch, and most of the uses seem to
    > > be of the form:
    > >
    > > increment ref count
    > > rcu_read_lock()
    > > do something
    > > rcu_read_unlock()
    > > decrement ref count
    > >
    > > Can't these cases rely solely on rcu_read_lock()? Why do you also
    > > need to increment the reference count in these cases?
    >
    > The problem is on module unload: is it possible for CPU1 to be
    > on "do something", and sleep, and, while it sleeps, CPU2 does
    > rmmod(lsm), so that by the time CPU1 stops sleeping, the code it
    > is executing has been freed?

    OK, but in the above case, "do something" cannot be sleeping, since
    it is under rcu_read_lock().

    > Because stacker won't remove the lsm from the list of modules
    > until mod->exit() is executed, and module_free(mod) happens
    > immediately after that, the above scenario seems possible.

    Right, if you have some other code path that sleeps (outside of
    rcu_read_lock(), right?), then you need the reference count for that
    code path. But the code paths that do not sleep should be able to
    dispense with the reference count, reducing the cache-line traffic.

    Thanx, Paul
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