On Tue, Dec 20, 2022 at 07:25:20PM +0800, Zqiang 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. 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expmask is obsolete by the time it invokes tick_dep_set_cpu(), and CPU1 is not guaranteed to see CPU2's store to -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))