ntp: Fix leap-second hrtimer livelock
Since commit 7dffa3c673 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: Sasha Levin <levinsasha928@gmail.com>
Diagnoised-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
This commit is contained in:
John Stultz
parent
57779dc2b3
commit
6b43ae8a61
3 changed files with 57 additions and 111 deletions
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@ -252,7 +252,7 @@ extern void ntp_clear(void);
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/* Returns how long ticks are at present, in ns / 2^NTP_SCALE_SHIFT. */
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extern u64 ntp_tick_length(void);
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extern void second_overflow(void);
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extern int second_overflow(unsigned long secs);
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extern int do_adjtimex(struct timex *);
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extern void hardpps(const struct timespec *, const struct timespec *);
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@ -34,8 +34,6 @@ unsigned long tick_nsec;
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static u64 tick_length;
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static u64 tick_length_base;
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static struct hrtimer leap_timer;
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#define MAX_TICKADJ 500LL /* usecs */
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#define MAX_TICKADJ_SCALED \
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(((MAX_TICKADJ * NSEC_PER_USEC) << NTP_SCALE_SHIFT) / NTP_INTERVAL_FREQ)
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@ -380,57 +378,6 @@ u64 ntp_tick_length(void)
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}
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/*
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* Leap second processing. If in leap-insert state at the end of the
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* day, the system clock is set back one second; if in leap-delete
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* state, the system clock is set ahead one second.
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*/
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static enum hrtimer_restart ntp_leap_second(struct hrtimer *timer)
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{
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enum hrtimer_restart res = HRTIMER_NORESTART;
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unsigned long flags;
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int leap = 0;
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spin_lock_irqsave(&ntp_lock, flags);
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switch (time_state) {
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case TIME_OK:
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break;
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case TIME_INS:
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leap = -1;
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time_state = TIME_OOP;
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printk(KERN_NOTICE
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"Clock: inserting leap second 23:59:60 UTC\n");
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hrtimer_add_expires_ns(&leap_timer, NSEC_PER_SEC);
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res = HRTIMER_RESTART;
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break;
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case TIME_DEL:
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leap = 1;
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time_tai--;
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time_state = TIME_WAIT;
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printk(KERN_NOTICE
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"Clock: deleting leap second 23:59:59 UTC\n");
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break;
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case TIME_OOP:
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time_tai++;
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time_state = TIME_WAIT;
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/* fall through */
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case TIME_WAIT:
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if (!(time_status & (STA_INS | STA_DEL)))
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time_state = TIME_OK;
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break;
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}
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spin_unlock_irqrestore(&ntp_lock, flags);
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/*
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* We have to call this outside of the ntp_lock to keep
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* the proper locking hierarchy
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*/
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if (leap)
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timekeeping_leap_insert(leap);
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return res;
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}
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/*
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* this routine handles the overflow of the microsecond field
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*
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@ -438,14 +385,58 @@ static enum hrtimer_restart ntp_leap_second(struct hrtimer *timer)
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* were provided by Dave Mills (Mills@UDEL.EDU) of NTP fame.
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* They were originally developed for SUN and DEC kernels.
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* All the kudos should go to Dave for this stuff.
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*
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* Also handles leap second processing, and returns leap offset
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*/
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void second_overflow(void)
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int second_overflow(unsigned long secs)
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{
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s64 delta;
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int leap = 0;
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unsigned long flags;
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spin_lock_irqsave(&ntp_lock, flags);
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/*
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* Leap second processing. If in leap-insert state at the end of the
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* day, the system clock is set back one second; if in leap-delete
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* state, the system clock is set ahead one second.
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*/
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switch (time_state) {
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case TIME_OK:
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if (time_status & STA_INS)
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time_state = TIME_INS;
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else if (time_status & STA_DEL)
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time_state = TIME_DEL;
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break;
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case TIME_INS:
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if (secs % 86400 == 0) {
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leap = -1;
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time_state = TIME_OOP;
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printk(KERN_NOTICE
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"Clock: inserting leap second 23:59:60 UTC\n");
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}
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break;
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case TIME_DEL:
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if ((secs + 1) % 86400 == 0) {
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leap = 1;
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time_tai--;
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time_state = TIME_WAIT;
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printk(KERN_NOTICE
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"Clock: deleting leap second 23:59:59 UTC\n");
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}
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break;
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case TIME_OOP:
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time_tai++;
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time_state = TIME_WAIT;
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break;
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case TIME_WAIT:
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if (!(time_status & (STA_INS | STA_DEL)))
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time_state = TIME_OK;
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break;
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}
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/* Bump the maxerror field */
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time_maxerror += MAXFREQ / NSEC_PER_USEC;
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if (time_maxerror > NTP_PHASE_LIMIT) {
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@ -481,8 +472,13 @@ void second_overflow(void)
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tick_length += (s64)(time_adjust * NSEC_PER_USEC / NTP_INTERVAL_FREQ)
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<< NTP_SCALE_SHIFT;
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time_adjust = 0;
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out:
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spin_unlock_irqrestore(&ntp_lock, flags);
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return leap;
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}
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#ifdef CONFIG_GENERIC_CMOS_UPDATE
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@ -544,27 +540,6 @@ static void notify_cmos_timer(void)
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static inline void notify_cmos_timer(void) { }
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#endif
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/*
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* Start the leap seconds timer:
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*/
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static inline void ntp_start_leap_timer(struct timespec *ts)
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{
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long now = ts->tv_sec;
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if (time_status & STA_INS) {
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time_state = TIME_INS;
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now += 86400 - now % 86400;
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hrtimer_start(&leap_timer, ktime_set(now, 0), HRTIMER_MODE_ABS);
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return;
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}
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if (time_status & STA_DEL) {
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time_state = TIME_DEL;
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now += 86400 - (now + 1) % 86400;
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hrtimer_start(&leap_timer, ktime_set(now, 0), HRTIMER_MODE_ABS);
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}
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}
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/*
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* Propagate a new txc->status value into the NTP state:
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@ -589,22 +564,6 @@ static inline void process_adj_status(struct timex *txc, struct timespec *ts)
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time_status &= STA_RONLY;
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time_status |= txc->status & ~STA_RONLY;
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switch (time_state) {
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case TIME_OK:
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ntp_start_leap_timer(ts);
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break;
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case TIME_INS:
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case TIME_DEL:
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time_state = TIME_OK;
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ntp_start_leap_timer(ts);
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case TIME_WAIT:
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if (!(time_status & (STA_INS | STA_DEL)))
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time_state = TIME_OK;
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break;
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case TIME_OOP:
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hrtimer_restart(&leap_timer);
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break;
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}
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}
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/*
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* Called with the xtime lock held, so we can access and modify
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@ -686,9 +645,6 @@ int do_adjtimex(struct timex *txc)
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(txc->tick < 900000/USER_HZ ||
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txc->tick > 1100000/USER_HZ))
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return -EINVAL;
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if (txc->modes & ADJ_STATUS && time_state != TIME_OK)
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hrtimer_cancel(&leap_timer);
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}
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if (txc->modes & ADJ_SETOFFSET) {
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@ -1010,6 +966,4 @@ __setup("ntp_tick_adj=", ntp_tick_adj_setup);
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void __init ntp_init(void)
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{
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ntp_clear();
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hrtimer_init(&leap_timer, CLOCK_REALTIME, HRTIMER_MODE_ABS);
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leap_timer.function = ntp_leap_second;
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}
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@ -184,18 +184,6 @@ static void timekeeping_update(bool clearntp)
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}
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void timekeeping_leap_insert(int leapsecond)
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{
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unsigned long flags;
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write_seqlock_irqsave(&timekeeper.lock, flags);
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timekeeper.xtime.tv_sec += leapsecond;
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timekeeper.wall_to_monotonic.tv_sec -= leapsecond;
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timekeeping_update(false);
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write_sequnlock_irqrestore(&timekeeper.lock, flags);
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}
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/**
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* timekeeping_forward_now - update clock to the current time
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*
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@ -969,9 +957,11 @@ static cycle_t logarithmic_accumulation(cycle_t offset, int shift)
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timekeeper.xtime_nsec += timekeeper.xtime_interval << shift;
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while (timekeeper.xtime_nsec >= nsecps) {
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int leap;
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timekeeper.xtime_nsec -= nsecps;
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timekeeper.xtime.tv_sec++;
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second_overflow();
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leap = second_overflow(timekeeper.xtime.tv_sec);
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timekeeper.xtime.tv_sec += leap;
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}
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/* Accumulate raw time */
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@ -1082,9 +1072,11 @@ static void update_wall_time(void)
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* xtime.tv_nsec isn't larger then NSEC_PER_SEC
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*/
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if (unlikely(timekeeper.xtime.tv_nsec >= NSEC_PER_SEC)) {
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int leap;
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timekeeper.xtime.tv_nsec -= NSEC_PER_SEC;
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timekeeper.xtime.tv_sec++;
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second_overflow();
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leap = second_overflow(timekeeper.xtime.tv_sec);
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timekeeper.xtime.tv_sec += leap;
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}
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timekeeping_update(false);
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