Re: [PATCH 2/2] rcu: Keep gpnum and completed fields synchronized

[Frederic Weisbecker <fweisbec@gmail.com>]

On Fri, Dec 10, 2010 at 03:02:00PM -0800, Paul E. McKenney wrote:
> On Fri, Dec 10, 2010 at 10:11:11PM +0100, Frederic Weisbecker wrote:
> > 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.
> 
> Also good, queued!
> 
> One issue -- this approach is vulnerable to overflow.  I therefore
> followed up with a patch that changes the condition to
> 
> 	if (ULONG_CMP_LT(rdp->gpnum, rdp->completed))
> 
> And I clearly need to make RCU defend itself against the scenario where
> a CPU stays in dyntick-idle mode long enough for the grace-period number
> to wrap halfway around its range of possible values.  Not a problem at
> the moment, and never will be for 64-bit systems, but...
> 
> I will fix that up.

Oh you're right of course. I did not think about possible overflows.

Now looking at ULONG_CMP_LT() definition, if it wraps more than halfways
we are screwed anyway. I suspect it won't ever happen, but it can. Perhaps
we need some watchguard code in note_new_gpnum() to fixup that corner case.


> 
> 							Thanx, Paul
> 
> > 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
> > ---
> >  kernel/rcutree.c |    9 +++++++++
> >  1 files changed, 9 insertions(+), 0 deletions(-)
> > 
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index 8c4ed60..2e16da3 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -683,6 +683,15 @@ __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;
> > +
> > +		/*
> >  		 * If another CPU handled our extended quiescent states and
> >  		 * we have no more grace period to complete yet, then stop
> >  		 * chasing quiescent states.
> > -- 
> > 1.7.3.2
> >