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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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