Re: [PATCH] rcu: Avoid to modify mask_ofl_ipi in sync_rcu_exp_select_node_cpus()

[Paul E. McKenney]

On Tue, Oct 08, 2019 at 06:35:45PM +0200, Marco Elver wrote:
> On Tue, 8 Oct 2019 at 18:30, Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
> >
> > On Tue, Oct 08, 2019 at 01:01:40PM +0800, Boqun Feng wrote:
> > > "mask_ofl_ipi" is used for iterate CPUs which IPIs are needed to send
> > > to, however in the IPI sending loop, "mask_ofl_ipi" along with another
> > > variable "mask_ofl_test" might also get modified to record which CPU's
> > > quiesent state can be reported by sync_rcu_exp_select_node_cpus(). Two
> > > variables seems to be redundant for such a propose, so this patch clean
> > > things a little by solely using "mask_ofl_test" for recording and
> > > "mask_ofl_ipi" for iteration. This would improve the readibility of the
> > > IPI sending loop in sync_rcu_exp_select_node_cpus().
> > >
> > > Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> > > ---
> >
> > Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> >
> > thanks,
> >
> >  - Joel
> 
> Acked-by: Marco Elver <elver@xxxxxxxxxx>
> 
> If this is the official patch for the fix to the KCSAN reported
> data-race, it'd be great to include the tag:
> Reported-by: syzbot+134336b86f728d6e55a0@xxxxxxxxxxxxxxxxxxxxxxxxx
> so the bot knows this was fixed.

I applied your Acked-by to both patches, but please let me know if you
intended something else.  Either way, thank you both for finding this
and for your testing!

							Thanx, Paul

> Thanks!
> -- Marco
> 
> > >  kernel/rcu/tree_exp.h | 13 ++++++-------
> > >  1 file changed, 6 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> > > index 69c5aa64fcfd..212470018752 100644
> > > --- a/kernel/rcu/tree_exp.h
> > > +++ b/kernel/rcu/tree_exp.h
> > > @@ -387,10 +387,10 @@ static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
> > >               }
> > >               ret = smp_call_function_single(cpu, rcu_exp_handler, NULL, 0);
> > >               put_cpu();
> > > -             if (!ret) {
> > > -                     mask_ofl_ipi &= ~mask;
> > > +             /* The CPU responses the IPI, and will report QS itself */
> > > +             if (!ret)
> > >                       continue;
> > > -             }
> > > +
> > >               /* Failed, raced with CPU hotplug operation. */
> > >               raw_spin_lock_irqsave_rcu_node(rnp, flags);
> > >               if ((rnp->qsmaskinitnext & mask) &&
> > > @@ -401,13 +401,12 @@ static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
> > >                       schedule_timeout_uninterruptible(1);
> > >                       goto retry_ipi;
> > >               }
> > > -             /* CPU really is offline, so we can ignore it. */
> > > -             if (!(rnp->expmask & mask))
> > > -                     mask_ofl_ipi &= ~mask;
> > > +             /* CPU really is offline, and we need its QS to pass GP. */
> > > +             if (rnp->expmask & mask)
> > > +                     mask_ofl_test |= mask;
> > >               raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> > >       }
> > >       /* Report quiescent states for those that went offline. */
> > > -     mask_ofl_test |= mask_ofl_ipi;
> > >       if (mask_ofl_test)
> > >               rcu_report_exp_cpu_mult(rnp, mask_ofl_test, false);
> > >  }
> > > --
> > > 2.23.0
> > >