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    SubjectRe: [PATCH RFC tip/core/rcu 15/20] rcu: Keep gpnum and completed fields synchronized
    On 12/18/2010 04:54 AM, Paul E. McKenney wrote:
    > From: Frederic Weisbecker <fweisbec@gmail.com>
    >
    > When a CPU that was in an extended quiescent state wakes
    > up and catches up with grace periods that remote CPUs
    > completed on its behalf, we update the completed field
    > but not the gpnum that keeps a stale value of a backward
    > grace period ID.
    >
    > Later, note_new_gpnum() will interpret the shift between
    > the local CPU and the node grace period ID as some new grace
    > period to handle and will then start to hunt quiescent state.
    >
    > But if every grace periods have already been completed, this
    > interpretation becomes broken. And we'll be stuck in clusters
    > of spurious softirqs because rcu_report_qs_rdp() will make
    > this broken state run into infinite loop.
    >
    > The solution, as suggested by Lai Jiangshan, is to ensure that
    > the gpnum and completed fields are well synchronized when we catch
    > up with completed grace periods on their behalf by other cpus.
    > This way we won't start noting spurious new grace periods.
    >
    > Suggested-by: Lai Jiangshan <laijs@cn.fujitsu.com>
    > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
    > Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    > Cc: Ingo Molnar <mingo@elte.hu>
    > Cc: Thomas Gleixner <tglx@linutronix.de>
    > Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    > Cc: Steven Rostedt <rostedt@goodmis.org
    > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    > ---
    > kernel/rcutree.c | 9 +++++++++
    > 1 files changed, 9 insertions(+), 0 deletions(-)
    >
    > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
    > index 916f42b..8105271 100644
    > --- a/kernel/rcutree.c
    > +++ b/kernel/rcutree.c
    > @@ -680,6 +680,15 @@ __rcu_process_gp_end(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_dat
    > rdp->completed = rnp->completed;
    >
    > /*
    > + * If we were in an extended quiescent state, we may have
    > + * missed some grace periods that others CPUs took care on
    > + * our behalf. Catch up with this state to avoid noting
    > + * spurious new grace periods.
    > + */
    > + if (rdp->completed > rdp->gpnum)
    > + rdp->gpnum = rdp->completed;

    Need to use ULONG_CMP_LT(rdp->gpnum, rdp->completed) instead.

    > +
    > + /*
    > * If another CPU handled our extended quiescent states and
    > * we have no more grace period to complete yet, then stop
    > * chasing quiescent states.



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