On Thu, Dec 22, 2022 at 09:48:14AM +0000, Zhang, Qiang1 wrote:
> > For the kernel bulit with CONFIG_NO_HZ_FULL enabled and the following
> > cpus is nohz_full cpus:
> >
> > CPU1 CPU2
> > rcu_report_exp_cpu_mult synchronize_rcu_expedited_wait
> > acquires rnp->lock mask = rnp->expmask;
> > for_each_leaf_node_cpu_mask(rnp, cpu, mask)
> > rnp->expmask = rnp->expmask & ~mask; rdp = per_cpu_ptr(&rcu_data, cpu1);
> > for_each_leaf_node_cpu_mask(rnp, cpu, mask)
> > rdp = per_cpu_ptr(&rcu_data, cpu1);
> > if (!rdp->rcu_forced_tick_exp)
> > continue; rdp->rcu_forced_tick_exp = true;
> >
> > tick_dep_set_cpu(cpu1, TICK_DEP_BIT_RCU_EXP);
> >
> > In the above scenario, after CPU1 reported the quiescent state, CPU1
> > misses the opportunity to clear the TICK_DEP_BIT_RCU_EXP bitmask, it
> > will not be cleared until the next expedited grace period starts and
> > the CPU1 quiescent state is reported again. during this window period,
> > the CPU1 whose tick can not be stopped, if CPU1 has only one runnable
> > task and this task has aggressive real-time response constraints, this
> > task may have one of the worst response times.
> >
> > Therefore, this commit add rnp->lock when set TICK_DEP_BIT_RCU_EXP
> > bitmask to fix this race.
> >
> > Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
> >
> >Good eyes, thank you!!!
> >
> >Queued for testing and further review as follows, as always, please check for errors.
> >
>
> It looks more clear now, thank you!
Thank you for checking them both!
Thanx, Paul
> Thanks
> Zqiang
>
> > Thanx, Paul
> >
> >------------------------------------------------------------------------
>
> commit acfe689f2e473fb59b6d2c95af5fe36198bb9a84
> Author: Zqiang <qiang1.zhang@xxxxxxxxx>
> Date: Tue Dec 20 19:25:20 2022 +0800
>
> rcu: Fix set/clear TICK_DEP_BIT_RCU_EXP bitmask race
>
> For kernels built with CONFIG_NO_HZ_FULL=y, the following scenario can result
> in the scheduling-clock interrupt remaining enabled on a holdout CPU after
> its quiescent state has been reported:
>
> CPU1 CPU2
> rcu_report_exp_cpu_mult synchronize_rcu_expedited_wait
> acquires rnp->lock mask = rnp->expmask;
> for_each_leaf_node_cpu_mask(rnp, cpu, mask)
> rnp->expmask = rnp->expmask & ~mask; rdp = per_cpu_ptr(&rcu_data, cpu1);
> for_each_leaf_node_cpu_mask(rnp, cpu, mask)
> rdp = per_cpu_ptr(&rcu_data, cpu1);
> if (!rdp->rcu_forced_tick_exp)
> continue; rdp->rcu_forced_tick_exp = true;
> tick_dep_set_cpu(cpu1, TICK_DEP_BIT_RCU_EXP);
>
> The problem is that CPU2's sampling of rnp->expmask is obsolete by the
> time it invokes tick_dep_set_cpu(), and CPU1 is not guaranteed to see
> CPU2's store to ->rcu_forced_tick_exp in time to clear it. And even if
> CPU1 does see that store, it might invoke tick_dep_clear_cpu() before
> CPU2 got around to executing its tick_dep_set_cpu(), which would still
> leave the victim CPU with its scheduler-clock tick running.
>
> Either way, an nohz_full real-time application running on the victim
> CPU would have its latency needlessly degraded.
>
> Note that expedited RCU grace periods look at context-tracking
> information, and so if the CPU is executing in nohz_full usermode
> throughout, that CPU cannot be victimized in this manner.
>
> This commit therefore causes synchronize_rcu_expedited_wait to hold
> the rcu_node structure's ->lock when checking for holdout CPUs, setting
> TICK_DEP_BIT_RCU_EXP, and invoking tick_dep_set_cpu(), thus preventing
> this race.
>
> Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
>
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h index 249c2967d9e6c..7cc4856da0817 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -594,6 +594,7 @@ static void synchronize_rcu_expedited_wait(void)
> struct rcu_data *rdp;
> struct rcu_node *rnp;
> struct rcu_node *rnp_root = rcu_get_root();
> + unsigned long flags;
>
> trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("startwait"));
> jiffies_stall = rcu_exp_jiffies_till_stall_check();
> @@ -602,17 +603,17 @@ static void synchronize_rcu_expedited_wait(void)
> if (synchronize_rcu_expedited_wait_once(1))
> return;
> rcu_for_each_leaf_node(rnp) {
> + raw_spin_lock_irqsave_rcu_node(rnp, flags);
> mask = READ_ONCE(rnp->expmask);
> for_each_leaf_node_cpu_mask(rnp, cpu, mask) {
> rdp = per_cpu_ptr(&rcu_data, cpu);
> if (rdp->rcu_forced_tick_exp)
> continue;
> rdp->rcu_forced_tick_exp = true;
> - preempt_disable();
> if (cpu_online(cpu))
> tick_dep_set_cpu(cpu, TICK_DEP_BIT_RCU_EXP);
> - preempt_enable();
> }
> + raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> }
> j = READ_ONCE(jiffies_till_first_fqs);
> if (synchronize_rcu_expedited_wait_once(j + HZ))
