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

[Marco Elver <elver@google.com>]

On Tue, 8 Oct 2019 at 18:30, Joel Fernandes <joel@joelfernandes.org> 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@gmail.com>
> > ---
>
> Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
>
> thanks,
>
>  - Joel

Acked-by: Marco Elver <elver@google.com>

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@syzkaller.appspotmail.com
so the bot knows this was fixed.

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
> >