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    Subject[PATCH RFC 2/8] RCU: Fix barriers
    Work in progress, not for inclusion.

    Fix rcu_barrier() to work properly in preemptive kernel environment.
    Also, the ordering of callback must be preserved while moving
    callbacks to another CPU during CPU hotplug.

    Signed-off-by: Dipankar Sarma <dipankar@in.ibm.com>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    ---

    rcuclassic.c | 2 +-
    rcupdate.c | 10 ++++++++++
    2 files changed, 11 insertions(+), 1 deletion(-)

    diff -urpNa -X dontdiff linux-2.6.22-a-splitclassic/kernel/rcuclassic.c linux-2.6.22-b-fixbarriers/kernel/rcuclassic.c
    --- linux-2.6.22-a-splitclassic/kernel/rcuclassic.c 2007-07-19 15:03:51.000000000 -0700
    +++ linux-2.6.22-b-fixbarriers/kernel/rcuclassic.c 2007-07-19 17:10:46.000000000 -0700
    @@ -349,9 +349,9 @@ static void __rcu_offline_cpu(struct rcu
    if (rcp->cur != rcp->completed)
    cpu_quiet(rdp->cpu, rcp);
    spin_unlock_bh(&rcp->lock);
    + rcu_move_batch(this_rdp, rdp->donelist, rdp->donetail);
    rcu_move_batch(this_rdp, rdp->curlist, rdp->curtail);
    rcu_move_batch(this_rdp, rdp->nxtlist, rdp->nxttail);
    - rcu_move_batch(this_rdp, rdp->donelist, rdp->donetail);
    }

    static void rcu_offline_cpu(int cpu)
    diff -urpNa -X dontdiff linux-2.6.22-a-splitclassic/kernel/rcupdate.c linux-2.6.22-b-fixbarriers/kernel/rcupdate.c
    --- linux-2.6.22-a-splitclassic/kernel/rcupdate.c 2007-07-19 14:19:03.000000000 -0700
    +++ linux-2.6.22-b-fixbarriers/kernel/rcupdate.c 2007-07-19 17:13:31.000000000 -0700
    @@ -115,7 +115,17 @@ void rcu_barrier(void)
    mutex_lock(&rcu_barrier_mutex);
    init_completion(&rcu_barrier_completion);
    atomic_set(&rcu_barrier_cpu_count, 0);
    + /*
    + * The queueing of callbacks in all CPUs must be atomic with
    + * respect to RCU, otherwise one CPU may queue a callback,
    + * wait for a grace period, decrement barrier count and call
    + * complete(), while other CPUs have not yet queued anything.
    + * So, we need to make sure that grace periods cannot complete
    + * until all the callbacks are queued.
    + */
    + rcu_read_lock();
    on_each_cpu(rcu_barrier_func, NULL, 0, 1);
    + rcu_read_unlock();
    wait_for_completion(&rcu_barrier_completion);
    mutex_unlock(&rcu_barrier_mutex);
    }
    -
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