On Wed, Jul 01, 2015 at 03:48:06PM +0200, Peter Zijlstra wrote:
> On Wed, Jul 01, 2015 at 06:41:44AM -0700, Paul E. McKenney wrote:
> > On Wed, Jul 01, 2015 at 12:05:21PM +0200, Peter Zijlstra wrote:
> > > On Tue, Jun 30, 2015 at 02:48:27PM -0700, Paul E. McKenney wrote:
> > > > rsp->gp_state = RCU_GP_WAIT_FQS;
> > > > ret =
> > > > wait_event_interruptible_timeout(rsp->gp_wq,
> > > > + ((gf = READ_ONCE(rsp->gp_flags)) &
> > > > + RCU_GP_FLAG_FQS) ||
> > > > + (!READ_ONCE(rnp->qsmask) &&
> > > > + !rcu_preempt_blocked_readers_cgp(rnp))
> > > > ||
> > > > + rcu_exp_gp_seq_done(rsp->exp_rsp,
> > > > + rsp->gp_exp_snap),
> > > > + j);
> > > > rsp->gp_state = RCU_GP_DONE_FQS;
> > >
> > > How can the GP be done if we timed out or got interrupted?
> >
> > If all the CPUs still blocking the grace period went idle, or in a
> > NO_HZ_FULL kernel, entered nohz_full userspace execution. Or, if
> > certain low-probability races happen, went offline.
>
> But what if none of those are true and we still timed out? You
> unconditionally grant the GP.
Say what???
I recheck the conditions and break out of the loop only if one or more
of the grace-period-end conditions is satisfied. If not, I invoke
rcu_gp_fqs() to do the scan to see if all remaining CPUs are idle,
in nohz_full userspace execution, or offline.
What am I mising here?
Thanx, Paul
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