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    SubjectRe: [tip:core/rcu] Revert "rcu: Decrease memory-barrier usage based on semi-formal proof"
    On Wed, May 25, 2011 at 06:13:10PM -0700, Paul E. McKenney wrote:
    > On Wed, May 25, 2011 at 03:49:25PM -0700, Yinghai Lu wrote:
    > > On 05/25/2011 03:34 PM, Paul E. McKenney wrote:
    > > > On Wed, May 25, 2011 at 03:15:50PM -0700, Yinghai Lu wrote:
    > > >>> There is a new branch yinghai.2011.05.24a on:
    > > >>>
    > > >>> git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-2.6-rcu.git
    > > >>>
    > > >>> Or will be as soon as kernel.org updates its mirrors.
    > > >>>
    > > >>> I am not sure I could call this "clean", but it does revert that commit
    > > >>> and 11 of the subsequent commits that depend on it. It does build,
    > > >>> and I will test it once my currently running tests complete.
    > > >>
    > > >> yes, with those revert, there is no delay in 10 times booting.
    > > >
    > > > Unfortunately, there are rcutorture test failures with the revert...
    > >
    > > confused.
    >
    > Given what I had to do to generate the revert, not exactly a surprise,
    > I am afraid. Just means that the resulting RCU sometimes fails to
    > wait for all pre-existing readers, and rcutorture catches it.
    >
    > > what is the next?
    >
    > 1. I send you a patch that I hope will fix the softlockup
    > you saw. I am testing this.
    >
    > 2. I am working on more detailed instrumentation, and will
    > send a patch on that.
    >
    > 3. If time allows, break down the operations RCU is doing
    > and test them in isolation.
    >
    > Other thoughts?

    And here is patch #1. Could you please try applying this on top of
    Peter Zijlstra's patch to see if it gets rid of the softlockups you saw?

    Thanx, Paul

    ------------------------------------------------------------------------

    rcu: Start RCU kthreads in TASK_INTERRUPTIBLE state

    Upon creation, kthreads are in TASK_UNINTERRUPTIBLE state, which can
    result in softlockup warnings. Because some of RCU's kthreads can
    legitimately be idle indefinitely, start them in TASK_INTERRUPTIBLE
    state in order to avoid those warnings.

    Suggested-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

    diff --git a/kernel/rcutree.c b/kernel/rcutree.c
    index a1a8bb6..40aab8d 100644
    --- a/kernel/rcutree.c
    +++ b/kernel/rcutree.c
    @@ -1647,6 +1647,7 @@ static int __cpuinit rcu_spawn_one_cpu_kthread(int cpu)
    if (IS_ERR(t))
    return PTR_ERR(t);
    kthread_bind(t, cpu);
    + set_task_state(t, TASK_INTERRUPTIBLE);
    per_cpu(rcu_cpu_kthread_cpu, cpu) = cpu;
    WARN_ON_ONCE(per_cpu(rcu_cpu_kthread_task, cpu) != NULL);
    per_cpu(rcu_cpu_kthread_task, cpu) = t;
    @@ -1754,6 +1755,7 @@ static int __cpuinit rcu_spawn_one_node_kthread(struct rcu_state *rsp,
    if (IS_ERR(t))
    return PTR_ERR(t);
    raw_spin_lock_irqsave(&rnp->lock, flags);
    + set_task_state(t, TASK_INTERRUPTIBLE);
    rnp->node_kthread_task = t;
    raw_spin_unlock_irqrestore(&rnp->lock, flags);
    sp.sched_priority = 99;
    diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
    index 049f278..a767b7d 100644
    --- a/kernel/rcutree_plugin.h
    +++ b/kernel/rcutree_plugin.h
    @@ -1295,6 +1295,7 @@ static int __cpuinit rcu_spawn_one_boost_kthread(struct rcu_state *rsp,
    if (IS_ERR(t))
    return PTR_ERR(t);
    raw_spin_lock_irqsave(&rnp->lock, flags);
    + set_task_state(t, TASK_INTERRUPTIBLE);
    rnp->boost_kthread_task = t;
    raw_spin_unlock_irqrestore(&rnp->lock, flags);
    sp.sched_priority = RCU_KTHREAD_PRIO;

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