[PATCH 2/2] arm64: smp: Tell RCU about CPUs that fail to come online

[Will Deacon]

Commit ce3d31ad3cac ("arm64/smp: Move rcu_cpu_starting() earlier") ensured
that RCU is informed early about incoming CPUs that might end up calling
into printk() before they are online. However, if such a CPU fails the
early CPU feature compatibility checks in check_local_cpu_capabilities(),
then it will be powered off or parked without informing RCU, leading to
an endless stream of stalls:

  | rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
  | rcu:	2-O...: (0 ticks this GP) idle=002/1/0x4000000000000000 softirq=0/0 fqs=2593
  | (detected by 0, t=5252 jiffies, g=9317, q=136)
  | Task dump for CPU 2:
  | task:swapper/2       state:R  running task     stack:    0 pid:    0 ppid:     1 flags:0x00000028
  | Call trace:
  | ret_from_fork+0x0/0x30

Ensure that the dying CPU invokes rcu_report_dead() prior to being powered
off or parked.

Cc: Qian Cai <cai@xxxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
Suggested-by: Qian Cai <cai@xxxxxxxxxx>
Signed-off-by: Will Deacon <will@xxxxxxxxxx>
---
 arch/arm64/kernel/smp.c | 1 +
 kernel/rcu/tree.c       | 2 +-
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index 09c96f57818c..18e9727d3f64 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -413,6 +413,7 @@ void cpu_die_early(void)
 
 	/* Mark this CPU absent */
 	set_cpu_present(cpu, 0);
+	rcu_report_dead(cpu);
 
 	if (IS_ENABLED(CONFIG_HOTPLUG_CPU)) {
 		update_cpu_boot_status(CPU_KILL_ME);
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 2a52f42f64b6..bd04b09b84b3 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4077,7 +4077,6 @@ void rcu_cpu_starting(unsigned int cpu)
 	smp_mb(); /* Ensure RCU read-side usage follows above initialization. */
 }
 
-#ifdef CONFIG_HOTPLUG_CPU
 /*
  * The outgoing function has no further need of RCU, so remove it from
  * the rcu_node tree's ->qsmaskinitnext bit masks.
@@ -4117,6 +4116,7 @@ void rcu_report_dead(unsigned int cpu)
 	rdp->cpu_started = false;
 }
 
+#ifdef CONFIG_HOTPLUG_CPU
 /*
  * The outgoing CPU has just passed through the dying-idle state, and we
  * are being invoked from the CPU that was IPIed to continue the offline
-- 
2.29.1.341.ge80a0c044ae-goog