[PATCH 5.10] rcu: Do not report strict GPs for outgoing CPUs

[PATCH 5.10] rcu: Do not report strict GPs for outgoing CPUs

[Zhen Lei]

From: "Paul E. McKenney" <paulmck@kernel.org>

[ Upstream commit bfb3aa735f82c8d98b32a669934ee7d6b346264d ]

An outgoing CPU is marked offline in a stop-machine handler and most
of that CPU's services stop at that point, including IRQ work queues.
However, that CPU must take another pass through the scheduler and through
a number of CPU-hotplug notifiers, many of which contain RCU readers.
In the past, these readers were not a problem because the outgoing CPU
has interrupts disabled, so that rcu_read_unlock_special() would not
be invoked, and thus RCU would never attempt to queue IRQ work on the
outgoing CPU.

This changed with the advent of the CONFIG_RCU_STRICT_GRACE_PERIOD
Kconfig option, in which rcu_read_unlock_special() is invoked upon exit
from almost all RCU read-side critical sections.  Worse yet, because
interrupts are disabled, rcu_read_unlock_special() cannot immediately
report a quiescent state and will therefore attempt to defer this
reporting, for example, by queueing IRQ work.  Which fails with a splat
because the CPU is already marked as being offline.

But it turns out that there is no need to report this quiescent state
because rcu_report_dead() will do this job shortly after the outgoing
CPU makes its final dive into the idle loop.  This commit therefore
makes rcu_read_unlock_special() refrain from queuing IRQ work onto
outgoing CPUs.

Fixes: 44bad5b3cca2 ("rcu: Do full report for .need_qs for strict GPs")
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Jann Horn <jannh@google.com>
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
---
 kernel/rcu/tree_plugin.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 6ed153f226b3925..244f32e98360fdf 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -628,7 +628,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
 			set_tsk_need_resched(current);
 			set_preempt_need_resched();
 			if (IS_ENABLED(CONFIG_IRQ_WORK) && irqs_were_disabled &&
-			    !rdp->defer_qs_iw_pending && exp) {
+			    !rdp->defer_qs_iw_pending && exp && cpu_online(rdp->cpu)) {
 				// Get scheduler to re-evaluate and call hooks.
 				// If !IRQ_WORK, FQS scan will eventually IPI.
 				init_irq_work(&rdp->defer_qs_iw,
-- 
2.26.0.106.g9fadedd

Re: [PATCH 5.10] rcu: Do not report strict GPs for outgoing CPUs

[Greg Kroah-Hartman]

On Tue, Feb 15, 2022 at 10:23:19AM +0800, Zhen Lei wrote:
> From: "Paul E. McKenney" <paulmck@kernel.org>
> 
> [ Upstream commit bfb3aa735f82c8d98b32a669934ee7d6b346264d ]
> 
> An outgoing CPU is marked offline in a stop-machine handler and most
> of that CPU's services stop at that point, including IRQ work queues.
> However, that CPU must take another pass through the scheduler and through
> a number of CPU-hotplug notifiers, many of which contain RCU readers.
> In the past, these readers were not a problem because the outgoing CPU
> has interrupts disabled, so that rcu_read_unlock_special() would not
> be invoked, and thus RCU would never attempt to queue IRQ work on the
> outgoing CPU.
> 
> This changed with the advent of the CONFIG_RCU_STRICT_GRACE_PERIOD
> Kconfig option, in which rcu_read_unlock_special() is invoked upon exit
> from almost all RCU read-side critical sections.  Worse yet, because
> interrupts are disabled, rcu_read_unlock_special() cannot immediately
> report a quiescent state and will therefore attempt to defer this
> reporting, for example, by queueing IRQ work.  Which fails with a splat
> because the CPU is already marked as being offline.
> 
> But it turns out that there is no need to report this quiescent state
> because rcu_report_dead() will do this job shortly after the outgoing
> CPU makes its final dive into the idle loop.  This commit therefore
> makes rcu_read_unlock_special() refrain from queuing IRQ work onto
> outgoing CPUs.
> 
> Fixes: 44bad5b3cca2 ("rcu: Do full report for .need_qs for strict GPs")
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> Cc: Jann Horn <jannh@google.com>
> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
> ---
>  kernel/rcu/tree_plugin.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 6ed153f226b3925..244f32e98360fdf 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -628,7 +628,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
>  			set_tsk_need_resched(current);
>  			set_preempt_need_resched();
>  			if (IS_ENABLED(CONFIG_IRQ_WORK) && irqs_were_disabled &&
> -			    !rdp->defer_qs_iw_pending && exp) {
> +			    !rdp->defer_qs_iw_pending && exp && cpu_online(rdp->cpu)) {
>  				// Get scheduler to re-evaluate and call hooks.
>  				// If !IRQ_WORK, FQS scan will eventually IPI.
>  				init_irq_work(&rdp->defer_qs_iw,
> -- 
> 2.26.0.106.g9fadedd
> 

Now queued up, thanks.

greg k-h