Re: [PATCH RFC tip/core/rcu 15/20] rcu: Keep gpnum and completed fields synchronized

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Dec 20, 2010 at 10:13:35AM +0800, Lai Jiangshan wrote:
> 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.

You are quite correct!  And the next patch in this series made exactly
that change.

							Thanx, Paul

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