[RFC PATCH] rtc: add a schedule point in 'rtc_timer_do_work'

[Xiongfeng Wang]

When I ran the syzkaller testsuite, it prints out the following calltrace.

[  138.350029] rcu: INFO: rcu_sched self-detected stall on CPU
[  138.354703] rcu: 	2-....: (104998 ticks this GP) idle=ece/1/0x4000000000000002 softirq=2825/2825 fqs=26250
[  138.357771] 	(t=105005 jiffies g=8033 q=1)
[  138.358952] NMI backtrace for cpu 2
[  138.359954] CPU: 2 PID: 170 Comm: kworker/2:1 Not tainted 5.2.0-514.55.6.9.x86_64 #64
[  138.362212] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.10.2-0-g5f4c7b1-prebuilt.qemu-project.org 04/01/2014
[  138.365818] Workqueue: events rtc_timer_do_work
[  138.367122] Call Trace:
[  138.367815]  <IRQ>
[  138.368420]  dump_stack+0xa7/0xf9
[  138.369372]  nmi_cpu_backtrace+0xc2/0xd0
[  138.370516]  ? lapic_can_unplug_cpu+0xa0/0xa0
[  138.371747]  nmi_trigger_cpumask_backtrace+0x14e/0x190
[  138.373201]  rcu_dump_cpu_stacks+0x97/0xc5
[  138.374376]  rcu_sched_clock_irq+0x819/0x8d0
[  138.375571]  ? tick_sched_do_timer+0xb0/0xb0
[  138.376787]  ? tick_sched_do_timer+0xb0/0xb0
[  138.377991]  update_process_times+0x2d/0x60
[  138.379176]  tick_sched_handle+0x2f/0x70
[  138.381088]  tick_sched_timer+0x41/0x90
[  138.382170]  __hrtimer_run_queues+0x11b/0x580
[  138.383386]  hrtimer_interrupt+0x11b/0x280
[  138.384585]  smp_apic_timer_interrupt+0x7a/0x230
[  138.385909]  apic_timer_interrupt+0xf/0x20
[  138.387066]  </IRQ>
[  138.387668] RIP: 0010:debug_lockdep_rcu_enabled+0x6/0x30
[  138.389149] Code: 1f 44 00 00 0f 1f 44 00 00 f0 ff 05 94 08 32 01 c3 0f 1f 00 0f 1f 44 00 00 f0 ff 0d 84 08 32 01 c3 0f 1f 00 8b 0d 2e 59 54 01 <31> c0 85 c9 74 21 8b 15 c2 b8 54 01 85 d2 74 17 65 48 8b 04 25 c0
[  138.394395] RSP: 0018:ffffc9000080fd30 EFLAGS: 00000203 ORIG_RAX: ffffffffffffff13
[  138.397117] RAX: 0000000000000000 RBX: ffff88813b77de10 RCX: 0000000000000002
[  138.399104] RDX: 0000000000000000 RSI: ffffffff81886439 RDI: ffff88813b77de10
[  138.401128] RBP: ffffc9000080fe38 R08: 0000000000000000 R09: 0000000000000000
[  138.403149] R10: 0000000000000001 R11: 0000000000000013 R12: ffff88813b77de98
[  138.405147] R13: ffff88813bab0c00 R14: 1a15e2eb5cd98000 R15: 000000003b9aca00
[  138.407118]  ? rtc_timer_do_work+0x349/0x710
[  138.408344]  rtc_timer_do_work+0x35c/0x710
[  138.409516]  ? graph_unlock+0x41/0x80
[  138.410591]  ? process_one_work+0x1d4/0x710
[  138.412062]  process_one_work+0x272/0x710
[  138.413193]  ? process_one_work+0x1d4/0x710
[  138.414376]  worker_thread+0x58/0x520
[  138.415405]  kthread+0x120/0x160
[  138.416337]  ? process_one_work+0x710/0x710
[  138.417486]  ? kthread_bind+0x20/0x20
[  138.418526]  ret_from_fork+0x24/0x30

After looking into the syzkaller log, I found out it is caused by the
following procedure. Firstly, enable the update interrupt through ioctl
'RTC_UIE_ON'. Secondly, set the rtc time to be a far further time
through ioctl 'RTC_SET_TIME'. This will cause we stuck in
'rtc_timer_do_work()' because the expiring time of 'UIE' is far less
than the current rtc time. This patch add a schedule point to avoid the
RCU calltrace.

Signed-off-by: Xiongfeng Wang <wangxiongfeng2@xxxxxxxxxx>
---
 drivers/rtc/interface.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/rtc/interface.c b/drivers/rtc/interface.c
index 72b7ddc4..0c8339d 100644
--- a/drivers/rtc/interface.c
+++ b/drivers/rtc/interface.c
@@ -911,6 +911,8 @@ void rtc_timer_do_work(struct work_struct *work)
 			timerqueue_add(&rtc->timerqueue, &timer->node);
 			trace_rtc_timer_enqueue(timer);
 		}
+
+		cond_resched();
 	}
 
 	/* Set next alarm */
-- 
1.7.12.4