rcu: Call touch_nmi_watchdog() while printing stall warnings

When RCU stall warning triggers, it can print out a lot of messages
while holding spinlocks.  If the console device is slow (e.g. an
actual or IPMI serial console), it may end up triggering NMI hard
lockup watchdog like the following.

*** CPU printking while holding RCU spinlock

  PID: 4149739  TASK: ffff881a46baa880  CPU: 13  COMMAND: "CPUThreadPool8"
   #0 [ffff881fff945e48] crash_nmi_callback at ffffffff8103f7d0
   FireflyTeam#1 [ffff881fff945e58] nmi_handle at ffffffff81020653
   FireflyTeam#2 [ffff881fff945eb0] default_do_nmi at ffffffff81020c36
   FireflyTeam#3 [ffff881fff945ed0] do_nmi at ffffffff81020d32
   FireflyTeam#4 [ffff881fff945ef0] end_repeat_nmi at ffffffff81956a7e
      [exception RIP: io_serial_in+21]
      RIP: ffffffff81630e55  RSP: ffff881fff943b88  RFLAGS: 00000002
      RAX: 000000000000ca00  RBX: ffffffff8230e188  RCX: 0000000000000000
      RDX: 00000000000002fd  RSI: 0000000000000005  RDI: ffffffff8230e188
      RBP: ffff881fff943bb0   R8: 0000000000000000   R9: ffffffff820cb3c4
      R10: 0000000000000019  R11: 0000000000002000  R12: 00000000000026e1
      R13: 0000000000000020  R14: ffffffff820cd398  R15: 0000000000000035
      ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0000
  --- <NMI exception stack> ---
   FireflyTeam#5 [ffff881fff943b88] io_serial_in at ffffffff81630e55
   FireflyTeam#6 [ffff881fff943b90] wait_for_xmitr at ffffffff8163175c
   FireflyTeam#7 [ffff881fff943bb8] serial8250_console_putchar at ffffffff816317dc
   FireflyTeam#8 [ffff881fff943bd8] uart_console_write at ffffffff8162ac00
   FireflyTeam#9 [ffff881fff943c08] serial8250_console_write at ffffffff81634691
  FireflyTeam#10 [ffff881fff943c80] univ8250_console_write at ffffffff8162f7c2
  FireflyTeam#11 [ffff881fff943c90] console_unlock at ffffffff810dfc55
  FireflyTeam#12 [ffff881fff943cf0] vprintk_emit at ffffffff810dffb5
  FireflyTeam#13 [ffff881fff943d50] vprintk_default at ffffffff810e01bf
  FireflyTeam#14 [ffff881fff943d60] vprintk_func at ffffffff810e1127
  FireflyTeam#15 [ffff881fff943d70] printk at ffffffff8119a8a4
  FireflyTeam#16 [ffff881fff943dd0] print_cpu_stall_info at ffffffff810eb78c
  rockchip-linux#17 [ffff881fff943e88] rcu_check_callbacks at ffffffff810ef133
  rockchip-linux#18 [ffff881fff943ee8] update_process_times at ffffffff810f3497
  rockchip-linux#19 [ffff881fff943f10] tick_sched_timer at ffffffff81103037
  rockchip-linux#20 [ffff881fff943f38] __hrtimer_run_queues at ffffffff810f3f38
  rockchip-linux#21 [ffff881fff943f88] hrtimer_interrupt at ffffffff810f442b

*** CPU triggering the hardlockup watchdog

  PID: 4149709  TASK: ffff88010f88c380  CPU: 26  COMMAND: "CPUThreadPool35"
   #0 [ffff883fff1059d0] machine_kexec at ffffffff8104a874
   FireflyTeam#1 [ffff883fff105a30] __crash_kexec at ffffffff811116cc
   FireflyTeam#2 [ffff883fff105af0] __crash_kexec at ffffffff81111795
   FireflyTeam#3 [ffff883fff105b08] panic at ffffffff8119a6ae
   FireflyTeam#4 [ffff883fff105b98] watchdog_overflow_callback at ffffffff81135dbd
   FireflyTeam#5 [ffff883fff105bb0] __perf_event_overflow at ffffffff81186866
   FireflyTeam#6 [ffff883fff105be8] perf_event_overflow at ffffffff81192bc4
   FireflyTeam#7 [ffff883fff105bf8] intel_pmu_handle_irq at ffffffff8100b265
   FireflyTeam#8 [ffff883fff105df8] perf_event_nmi_handler at ffffffff8100489f
   FireflyTeam#9 [ffff883fff105e58] nmi_handle at ffffffff81020653
  FireflyTeam#10 [ffff883fff105eb0] default_do_nmi at ffffffff81020b94
  FireflyTeam#11 [ffff883fff105ed0] do_nmi at ffffffff81020d32
  FireflyTeam#12 [ffff883fff105ef0] end_repeat_nmi at ffffffff81956a7e
      [exception RIP: queued_spin_lock_slowpath+248]
      RIP: ffffffff810da958  RSP: ffff883fff103e68  RFLAGS: 00000046
      RAX: 0000000000000000  RBX: 0000000000000046  RCX: 00000000006d0000
      RDX: ffff883fff49a950  RSI: 0000000000d10101  RDI: ffffffff81e54300
      RBP: ffff883fff103e80   R8: ffff883fff11a950   R9: 0000000000000000
      R10: 000000000e5873ba  R11: 000000000000010f  R12: ffffffff81e54300
      R13: 0000000000000000  R14: ffff88010f88c380  R15: ffffffff81e54300
      ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
  --- <NMI exception stack> ---
  FireflyTeam#13 [ffff883fff103e68] queued_spin_lock_slowpath at ffffffff810da958
  FireflyTeam#14 [ffff883fff103e70] _raw_spin_lock_irqsave at ffffffff8195550b
  FireflyTeam#15 [ffff883fff103e88] rcu_check_callbacks at ffffffff810eed18
  FireflyTeam#16 [ffff883fff103ee8] update_process_times at ffffffff810f3497
  rockchip-linux#17 [ffff883fff103f10] tick_sched_timer at ffffffff81103037
  rockchip-linux#18 [ffff883fff103f38] __hrtimer_run_queues at ffffffff810f3f38
  rockchip-linux#19 [ffff883fff103f88] hrtimer_interrupt at ffffffff810f442b
  --- <IRQ stack> ---

Avoid spuriously triggering NMI hardlockup watchdog by touching it
from the print functions.  show_state_filter() shares the same problem
and solution.

v2: Relocate the comment to where it belongs.

Signed-off-by: Tejun Heo <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>

kernel/rcu/tree_plugin.h

@@ -560,8 +560,14 @@ static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
 	}
 	t = list_entry(rnp->gp_tasks->prev,
 		       struct task_struct, rcu_node_entry);
-	list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry)
+	list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
+		/*
+		 * We could be printing a lot while holding a spinlock.
+		 * Avoid triggering hard lockup.
+		 */
+		touch_nmi_watchdog();
 		sched_show_task(t);
+	}
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 }
 
@@ -1677,6 +1683,12 @@ static void print_cpu_stall_info(struct rcu_state *rsp, int cpu)
 	char *ticks_title;
 	unsigned long ticks_value;
 
+	/*
+	 * We could be printing a lot while holding a spinlock.  Avoid
+	 * triggering hard lockup.
+	 */
+	touch_nmi_watchdog();
+
 	if (rsp->gpnum == rdp->gpnum) {
 		ticks_title = "ticks this GP";
 		ticks_value = rdp->ticks_this_gp;