rcu: Do not report strict GPs for outgoing CPUs

mainline inclusion
from mainline-v5.12-rc1
commit bfb3aa73
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I4UAEG
CVE: NA

-------------------------------------------------------------------------

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: 44bad5b3 ("rcu: Do full report for .need_qs for strict GPs")
Signed-off-by: NPaul E. McKenney <paulmck@kernel.org>
Cc: Jann Horn <jannh@google.com>
Signed-off-by: NZhen Lei <thunder.leizhen@huawei.com>
Reviewed-by: NCheng Jian <cj.chengjian@huawei.com>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

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,