The following commit has been merged into the core/rcu branch of tip:
Commit-ID: bfb3aa735f82c8d98b32a669934ee7d6b346264d
Gitweb:
https://git.kernel.org/tip/bfb3aa735f82c8d98b32a669934ee7d6b346264d
Author: Paul E. McKenney <paul...@kernel.org>
AuthorDate: Fri, 30 Oct 2020 13:11:24 -07:00
Committer: Paul E. McKenney <paul...@kernel.org>
CommitterDate: Thu, 19 Nov 2020 19:37:17 -08:00
rcu: Do not report strict GPs for outgoing CPUs
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 <paul...@kernel.org>
Cc: Jann Horn <ja...@google.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 fd8a52e..7e291ce 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,
