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Commit 6b43ae8a authored by John Stultz's avatar John Stultz
Browse files

ntp: Fix leap-second hrtimer livelock 



Since commit 7dffa3c6 the ntp
subsystem has used an hrtimer for triggering the leapsecond
adjustment. However, this can cause a potential livelock.

Thomas diagnosed this as the following pattern:
CPU 0                                                    CPU 1
do_adjtimex()
  spin_lock_irq(&ntp_lock);
    process_adjtimex_modes();				 timer_interrupt()
      process_adj_status();                                do_timer()
        ntp_start_leap_timer();                             write_lock(&xtime_lock);
          hrtimer_start();                                  update_wall_time();
             hrtimer_reprogram();                            ntp_tick_length()
               tick_program_event()                            spin_lock(&ntp_lock);
                 clockevents_program_event()
		   ktime_get()
                     seq = req_seqbegin(xtime_lock);

This patch tries to avoid the problem by reverting back to not using
an hrtimer to inject leapseconds, and instead we handle the leapsecond
processing in the second_overflow() function.

The downside to this change is that on systems that support highres
timers, the leap second processing will occur on a HZ tick boundary,
(ie: ~1-10ms, depending on HZ)  after the leap second instead of
possibly sooner (~34us in my tests w/ x86_64 lapic).

This patch applies on top of tip/timers/core.

CC: Sasha Levin <levinsasha928@gmail.com>
CC: Thomas Gleixner <tglx@linutronix.de>
Reported-by: default avatarSasha Levin <levinsasha928@gmail.com>
Diagnoised-by: default avatarThomas Gleixner <tglx@linutronix.de>
Tested-by: default avatarSasha Levin <levinsasha928@gmail.com>
Signed-off-by: default avatarJohn Stultz <john.stultz@linaro.org>
parent 57779dc2

include/linux/timex.h

+1 −1
Original line number Diff line number Diff line
@@ -252,7 +252,7 @@ extern void ntp_clear(void); 
/* Returns how long ticks are at present, in ns / 2^NTP_SCALE_SHIFT. */

extern u64 ntp_tick_length(void);



extern void second_overflow(void);

extern int second_overflow(unsigned long secs);

extern int do_adjtimex(struct timex *);

extern void hardpps(const struct timespec *, const struct timespec *);

kernel/time/ntp.c

+41 −87
Original line number Diff line number Diff line
@@ -34,8 +34,6 @@ unsigned long tick_nsec; 
static u64			tick_length;

static u64			tick_length_base;



static struct hrtimer		leap_timer;



#define MAX_TICKADJ		500LL		/* usecs */

#define MAX_TICKADJ_SCALED \

	(((MAX_TICKADJ * NSEC_PER_USEC) << NTP_SCALE_SHIFT) / NTP_INTERVAL_FREQ)
@@ -381,70 +379,63 @@ u64 ntp_tick_length(void) 




/*

 * Leap second processing. If in leap-insert state at the end of the

 * day, the system clock is set back one second; if in leap-delete

 * state, the system clock is set ahead one second.

 * this routine handles the overflow of the microsecond field

 *

 * The tricky bits of code to handle the accurate clock support

 * were provided by Dave Mills (Mills@UDEL.EDU) of NTP fame.

 * They were originally developed for SUN and DEC kernels.

 * All the kudos should go to Dave for this stuff.

 *

 * Also handles leap second processing, and returns leap offset

 */

static enum hrtimer_restart ntp_leap_second(struct hrtimer *timer)

int second_overflow(unsigned long secs)

{

	enum hrtimer_restart res = HRTIMER_NORESTART;

	unsigned long flags;

	s64 delta;

	int leap = 0;

	unsigned long flags;



	spin_lock_irqsave(&ntp_lock, flags);



	/*

	 * Leap second processing. If in leap-insert state at the end of the

	 * day, the system clock is set back one second; if in leap-delete

	 * state, the system clock is set ahead one second.

	 */

	switch (time_state) {

	case TIME_OK:

		if (time_status & STA_INS)

			time_state = TIME_INS;

		else if (time_status & STA_DEL)

			time_state = TIME_DEL;

		break;

	case TIME_INS:

		if (secs % 86400 == 0) {

			leap = -1;

			time_state = TIME_OOP;

			printk(KERN_NOTICE

				"Clock: inserting leap second 23:59:60 UTC\n");

		hrtimer_add_expires_ns(&leap_timer, NSEC_PER_SEC);

		res = HRTIMER_RESTART;

		}

		break;

	case TIME_DEL:

		if ((secs + 1) % 86400 == 0) {

			leap = 1;

			time_tai--;

			time_state = TIME_WAIT;

			printk(KERN_NOTICE

				"Clock: deleting leap second 23:59:59 UTC\n");

		}

		break;

	case TIME_OOP:

		time_tai++;

		time_state = TIME_WAIT;

		/* fall through */

		break;



	case TIME_WAIT:

		if (!(time_status & (STA_INS | STA_DEL)))

			time_state = TIME_OK;

		break;

	}

	spin_unlock_irqrestore(&ntp_lock, flags);



	/*

	 * We have to call this outside of the ntp_lock to keep

	 * the proper locking hierarchy

	 */

	if (leap)

		timekeeping_leap_insert(leap);



	return res;

}



/*

 * this routine handles the overflow of the microsecond field

 *

 * The tricky bits of code to handle the accurate clock support

 * were provided by Dave Mills (Mills@UDEL.EDU) of NTP fame.

 * They were originally developed for SUN and DEC kernels.

 * All the kudos should go to Dave for this stuff.

 */

void second_overflow(void)

{

	s64 delta;

	unsigned long flags;



	spin_lock_irqsave(&ntp_lock, flags);



	/* Bump the maxerror field */

	time_maxerror += MAXFREQ / NSEC_PER_USEC;
@@ -481,8 +472,13 @@ void second_overflow(void) 
	tick_length += (s64)(time_adjust * NSEC_PER_USEC / NTP_INTERVAL_FREQ)

							 << NTP_SCALE_SHIFT;

	time_adjust = 0;







out:

	spin_unlock_irqrestore(&ntp_lock, flags);



	return leap;

}



#ifdef CONFIG_GENERIC_CMOS_UPDATE
@@ -544,27 +540,6 @@ static void notify_cmos_timer(void) 
static inline void notify_cmos_timer(void) { }

#endif



/*

 * Start the leap seconds timer:

 */

static inline void ntp_start_leap_timer(struct timespec *ts)

{

	long now = ts->tv_sec;



	if (time_status & STA_INS) {

		time_state = TIME_INS;

		now += 86400 - now % 86400;

		hrtimer_start(&leap_timer, ktime_set(now, 0), HRTIMER_MODE_ABS);



		return;

	}



	if (time_status & STA_DEL) {

		time_state = TIME_DEL;

		now += 86400 - (now + 1) % 86400;

		hrtimer_start(&leap_timer, ktime_set(now, 0), HRTIMER_MODE_ABS);

	}

}



/*

 * Propagate a new txc->status value into the NTP state:
@@ -589,22 +564,6 @@ static inline void process_adj_status(struct timex *txc, struct timespec *ts) 
	time_status &= STA_RONLY;

	time_status |= txc->status & ~STA_RONLY;



	switch (time_state) {

	case TIME_OK:

		ntp_start_leap_timer(ts);

		break;

	case TIME_INS:

	case TIME_DEL:

		time_state = TIME_OK;

		ntp_start_leap_timer(ts);

	case TIME_WAIT:

		if (!(time_status & (STA_INS | STA_DEL)))

			time_state = TIME_OK;

		break;

	case TIME_OOP:

		hrtimer_restart(&leap_timer);

		break;

	}

}

/*

 * Called with the xtime lock held, so we can access and modify
@@ -686,9 +645,6 @@ int do_adjtimex(struct timex *txc) 
		    (txc->tick <  900000/USER_HZ ||

		     txc->tick > 1100000/USER_HZ))

			return -EINVAL;



		if (txc->modes & ADJ_STATUS && time_state != TIME_OK)

			hrtimer_cancel(&leap_timer);

	}



	if (txc->modes & ADJ_SETOFFSET) {
@@ -1010,6 +966,4 @@ __setup("ntp_tick_adj=", ntp_tick_adj_setup); 
void __init ntp_init(void)

{

	ntp_clear();

	hrtimer_init(&leap_timer, CLOCK_REALTIME, HRTIMER_MODE_ABS);

	leap_timer.function = ntp_leap_second;

}

kernel/time/timekeeping.c

+6 −14
Original line number Diff line number Diff line
@@ -184,18 +184,6 @@ static void timekeeping_update(bool clearntp) 
}





void timekeeping_leap_insert(int leapsecond)

{

	unsigned long flags;



	write_seqlock_irqsave(&timekeeper.lock, flags);

	timekeeper.xtime.tv_sec += leapsecond;

	timekeeper.wall_to_monotonic.tv_sec -= leapsecond;

	timekeeping_update(false);

	write_sequnlock_irqrestore(&timekeeper.lock, flags);



}



/**

 * timekeeping_forward_now - update clock to the current time

 *
@@ -969,9 +957,11 @@ static cycle_t logarithmic_accumulation(cycle_t offset, int shift) 


	timekeeper.xtime_nsec += timekeeper.xtime_interval << shift;

	while (timekeeper.xtime_nsec >= nsecps) {

		int leap;

		timekeeper.xtime_nsec -= nsecps;

		timekeeper.xtime.tv_sec++;

		second_overflow();

		leap = second_overflow(timekeeper.xtime.tv_sec);

		timekeeper.xtime.tv_sec += leap;

	}



	/* Accumulate raw time */
@@ -1082,9 +1072,11 @@ static void update_wall_time(void) 
	 * xtime.tv_nsec isn't larger then NSEC_PER_SEC

	 */

	if (unlikely(timekeeper.xtime.tv_nsec >= NSEC_PER_SEC)) {

		int leap;

		timekeeper.xtime.tv_nsec -= NSEC_PER_SEC;

		timekeeper.xtime.tv_sec++;

		second_overflow();

		leap = second_overflow(timekeeper.xtime.tv_sec);

		timekeeper.xtime.tv_sec += leap;

	}



	timekeeping_update(false);