ntp: Fix leap-second hrtimer livelock

Since commit 7dffa3c673fbcf835cd7be80bb4aec8ad3f51168 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>

diff --git a/include/linux/timex.h b/include/linux/timex.h
index b75e1864ed19c9e151fb0b17f9ebc7bdf4a39b09..99bc88b1fc02734a30619038d2f9a7f430b4a951 100644 (file)
--- a/include/linux/timex.h
+++ b/include/linux/timex.h
@@ -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 *);
 

diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
index 6e039b144daf0359ecaffcbc00227a7233fd0dd3..3d17ebd47fa252044b69a2b339e340148dcf9f44 100644 (file)
--- a/kernel/time/ntp.c
+++ b/kernel/time/ntp.c
@@ -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:
-               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;
+               if (secs % 86400 == 0) {
+                       leap = -1;
+                       time_state = TIME_OOP;
+                       printk(KERN_NOTICE
+                               "Clock: inserting leap second 23:59:60 UTC\n");
+               }
                break;
        case TIME_DEL:
-               leap = 1;
-               time_tai--;
-               time_state = TIME_WAIT;
-               printk(KERN_NOTICE
-                       "Clock: deleting leap second 23:59:59 UTC\n");
+               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;
 }

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index b53da5ecbea2e690490ed6395aa47e45bf0bfda0..5d76e09ddd3d04d1748e0ece2970d090c5b2621f 100644 (file)
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -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);