Messages in this thread |  | | | Date | Sat, 11 Dec 2010 01:00:39 +0100 | | From | Frederic Weisbecker <> | | Subject | Re: [PATCH 2/2] rcu: Keep gpnum and completed fields synchronized |
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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 > >
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