[PATCH] rtc: adjust the next alarm expiring time to avoid a softlockups.

[Jiwei Sun <jiwei.sun@windriver.com>]

When run the following test case,

/*
 * This expects the new RTC class driver framework, working with
 * clocks that will often not be clones of what the PC-AT had.
 * Use the command line to specify another RTC if you need one.
 */
static const char default_rtc[] = "/dev/rtc0";
int main(int argc, char **argv)
{
	int i, fd, retval;
	unsigned long data;
	struct rtc_time rtc_tm;
	const char *rtc = default_rtc;

	fd = open(rtc, O_RDONLY);
	if (fd == -1) {
		perror(rtc);
		exit(errno);
	}

	retval = ioctl(fd, RTC_RD_TIME, &rtc_tm);
	if (retval == -1) {
		perror("RTC_RD_TIME ioctl");
		exit(errno);
	}

	// Setting time 1970
	rtc_tm.tm_year = 70;
	retval = ioctl(fd, RTC_SET_TIME, &rtc_tm);
	if (retval == -1) {
		perror("RTC_SET_TIME ioctl");
		exit(errno);
	}

	/* Turn on update interrupts (one per second) */
	retval = ioctl(fd, RTC_UIE_ON, 0);
	if (retval == -1) {
		if (errno == ENOTTY) {
			fprintf(stderr,
				"\n...Update IRQs not supported.\n");
			goto done;
		}
		perror("RTC_UIE_ON ioctl");
		exit(errno);
	}

	fprintf(stderr, "Counting 5 update (1/sec) interrupts from reading %s:",
			rtc);
	fflush(stderr);

	for (i=1; i<6; i++) {
		if (i == 3) {
			// Setting new time 2030
			rtc_tm.tm_year = 130;
			retval = ioctl(fd, RTC_SET_TIME, &rtc_tm);
			if (retval == -1) {
				perror("RTC_SET_TIME ioctl");
				exit(errno);
			}
		}

		/* This read will block */
		retval = read(fd, &data, sizeof(unsigned long));
		if (retval == -1) {
			perror("read");
			exit(errno);
		}
		fprintf(stderr, " %d, 0x%lx",i, data);
		fflush(stderr);
	}

	/* Turn off update interrupts */
	retval = ioctl(fd, RTC_UIE_OFF, 0);
	if (retval == -1) {
		perror("RTC_UIE_OFF ioctl");
		exit(errno);
	}

done:
	fprintf(stderr, "\n\n\t\t\t *** Test complete ***\n");

	close(fd);

	return 0;
}

there will be the following call trace in the preempt kernel.
If the kernel is non-preempt, the softlockups will happen.

[   29.928336] INFO: rcu_sched self-detected stall on CPU
[   29.928336] 	2-....: (20934 ticks this GP) idle=f4a/1/4611686018427387906 softirq=754/754 fqs=4704
[   29.928336] 	 (t=21000 jiffies g=55 c=54 q=42)
[   29.928336] NMI backtrace for cpu 2
[   29.928336] CPU: 2 PID: 49 Comm: kworker/2:1 Not tainted 4.16.0 #5
[   29.928336] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
[   29.928336] Workqueue: events rtc_timer_do_work
[   29.928336] Call Trace:
[   29.928336]  <IRQ>
[   29.928336]  dump_stack+0x46/0x64
[   29.928336]  nmi_cpu_backtrace+0xba/0xc0
[   29.928336]  ? lapic_can_unplug_cpu+0x90/0x90
[   29.928336]  nmi_trigger_cpumask_backtrace+0x95/0xd0
[   29.928336]  rcu_dump_cpu_stacks+0x84/0xb4
[   29.928336]  rcu_check_callbacks+0x6b5/0x800
[   29.928336]  ? tick_sched_do_timer+0x40/0x40
[   29.928336]  update_process_times+0x23/0x50
[   29.928336]  tick_sched_handle+0x21/0x50
[   29.928336]  tick_sched_timer+0x2f/0x60
[   29.928336]  __hrtimer_run_queues+0xe4/0x240
[   29.928336]  hrtimer_interrupt+0x11d/0x270
[   29.928336]  smp_apic_timer_interrupt+0x51/0x120
[   29.928336]  apic_timer_interrupt+0xf/0x20
[   29.928336]  </IRQ>
[   29.928336] RIP: 0010:_raw_spin_unlock_irqrestore+0x5/0x10
[   29.928336] RSP: 0018:ffff9db0801abd50 EFLAGS: 00000286 ORIG_RAX: ffffffffffffff12
[   29.928336] RAX: 0000000000000286 RBX: ffff8dfb869fe800 RCX: 00000000cbfa5590
[   29.928336] RDX: 0000000000000000 RSI: 0000000000000286 RDI: ffff8dfb869febb0
[   29.928336] RBP: ffff8dfb869febb0 R08: 0000000000000000 R09: ffff9db0801abcf8
[   29.928336] R10: 0000000000000010 R11: 0000000000000001 R12: 0000000000000286
[   29.928336] R13: 0000000000000001 R14: ffff8dfb869fe800 R15: ffff8dfb869fec20
[   29.928336]  rtc_handle_legacy_irq+0x79/0xb0
[   29.928336]  rtc_timer_do_work+0xcc/0x200
[   29.928336]  process_one_work+0x136/0x330
[   29.928336]  worker_thread+0x3f/0x3b0
[   29.928336]  kthread+0xf0/0x130
[   29.928336]  ? process_one_work+0x330/0x330
[   29.928336]  ? kthread_bind+0x10/0x10
[   29.928336]  ? call_usermodehelper_exec_async+0x10f/0x110
[   29.928336]  ret_from_fork+0x35/0x40
[   29.934096] INFO: rcu_sched detected stalls on CPUs/tasks:
[   29.934096] 	2-....: (20935 ticks this GP) idle=f4a/1/4611686018427387904 softirq=754/754 fqs=4704
[   29.934096] 	(detected by 3, t=21019 jiffies, g=55, c=54, q=42)
[   29.934096] Sending NMI from CPU 3 to CPUs 2:
[   29.948423] NMI backtrace for cpu 2
[   29.948423] CPU: 2 PID: 49 Comm: kworker/2:1 Not tainted 4.16.0 #5
[   29.948423] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
[   29.952553] Workqueue: events rtc_timer_do_work
[   29.952553] RIP: 0010:__wake_up_common+0x0/0x120
[   29.952553] RSP: 0018:ffff9db0801abcf0 EFLAGS: 00000046
[   29.952553] RAX: 0000000000000292 RBX: ffff8dfb869feb88 RCX: 0000000000000000
[   29.952553] RDX: 0000000000000001 RSI: 0000000000000001 RDI: ffff8dfb869feb88
[   29.952553] RBP: 0000000000000001 R08: 0000000000000000 R09: ffff9db0801abcf8
[   29.952553] R10: 0000000000000010 R11: 0000000000000001 R12: 0000000000000292
[   29.952553] R13: 0000000000000001 R14: 0000000000000000 R15: 0000000000000000
[   29.952553] FS:  0000000000000000(0000) GS:ffff8dfb87d00000(0000) knlGS:0000000000000000
[   29.952553] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   29.952553] CR2: 00007fff0ee7a9f0 CR3: 00000000062b2000 CR4: 00000000000006e0
[   29.952553] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   29.952553] DR3: 0000000000000000 DR6: 0000000000000000 DR7: 0000000000000000
[   29.952553] Call Trace:
[   29.952553]  __wake_up_common_lock+0x65/0x90
[   29.952553]  rtc_handle_legacy_irq+0x91/0xb0
[   29.952553]  rtc_timer_do_work+0xcc/0x200
[   29.952553]  process_one_work+0x136/0x330
[   29.952553]  worker_thread+0x3f/0x3b0
[   29.952553]  kthread+0xf0/0x130
[   29.952553]  ? process_one_work+0x330/0x330
[   29.952553]  ? kthread_bind+0x10/0x10
[   29.952553]  ? call_usermodehelper_exec_async+0x10f/0x110
[   29.952553]  ret_from_fork+0x35/0x40
[   29.952553] Code: 90 90 90 90 90 90 90 90 90 90 90 48 8d 47 08 c7 07 00 00 00 00 48 89 47 08 48 89 47 10 c3 66 66 66 66 2e 0f 1f 84 00 00 00 00 00 <41> 57 41 56 4d 89 c6 41 55 41 54 55 4c 89 cd 53 48 83 ec 10 4d
[   29.952553] INFO: NMI handler (nmi_cpu_backtrace_handler) took too long to run: 6.037 msecs

The root cause is as following.
When the hardware clock's time is modified later than the periodic
alarm clock's expire time, the rtc_timer_do_work's loop may spend
a long time to process.
In order to avoid the case, we can adjust the next expire time to
equating to "now + period".

Signed-off-by: Jiwei Sun <jiwei.sun@windriver.com>
---
 drivers/rtc/interface.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/rtc/interface.c b/drivers/rtc/interface.c
index 672b192..d76eceb 100644
--- a/drivers/rtc/interface.c
+++ b/drivers/rtc/interface.c
@@ -877,6 +877,9 @@ void rtc_timer_do_work(struct work_struct *work)
 
 		/* Re-add/fwd periodic timers */
 		if (ktime_to_ns(timer->period)) {
+			if (now - next->expires > timer->period)
+				timer->node.expires = now;
+
 			timer->node.expires = ktime_add(timer->node.expires,
 							timer->period);
 			timer->enabled = 1;
-- 
1.9.1