ntp: Fix leap-second hrtimer livelock

This is a backport of 6b43ae8a619d17c4935c3320d2ef9e92bdeed05d

This should have been backported when it was commited, but I
mistook the problem as requiring the ntp_lock changes
that landed in 3.4 in order for it to occur.

Unfortunately the same issue can happen (with only one cpu)
as follows:
do_adjtimex()
 write_seqlock_irq(&xtime_lock);
  process_adjtimex_modes()
   process_adj_status()
    ntp_start_leap_timer()
     hrtimer_start()
      hrtimer_reprogram()
       tick_program_event()
        clockevents_program_event()
         ktime_get()
          seq = req_seqbegin(xtime_lock); [DEADLOCK]

This deadlock will no always occur, as it requires the
leap_timer to force a hrtimer_reprogram which only happens
if its set and there's no sooner timer to expire.

NOTE: This patch, being faithful to the original commit,
introduces a bug (we don't update wall_to_monotonic),
which will be resovled by backporting a following fix.

Original commit message below:

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>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

2 files changed, 47 insertions, 93 deletions

diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
index 4b85a7a7252..4508f7f68a7 100644
--- a/kernel/time/ntp.c
+++ b/kernel/time/ntp.c
@@ -31,8 +31,6 @@ unsigned long			tick_nsec;
 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)
@@ -350,60 +348,60 @@ void ntp_clear(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;
-
-        write_seqlock(&xtime_lock);
+        int leap = 0;
+        s64 delta;
 
+        /*
+         * 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:
-                timekeeping_leap_insert(-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:
-                timekeeping_leap_insert(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;
         }
 
-        write_sequnlock(&xtime_lock);
-
-        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;
 
         /* Bump the maxerror field */
         time_maxerror += MAXFREQ / NSEC_PER_USEC;
@@ -423,23 +421,25 @@ void second_overflow(void)
         pps_dec_valid();
 
         if (!time_adjust)
-                return;
+                goto out;
 
         if (time_adjust > MAX_TICKADJ) {
                 time_adjust -= MAX_TICKADJ;
                 tick_length += MAX_TICKADJ_SCALED;
-                return;
+                goto out;
         }
 
         if (time_adjust < -MAX_TICKADJ) {
                 time_adjust += MAX_TICKADJ;
                 tick_length -= MAX_TICKADJ_SCALED;
-                return;
+                goto out;
         }
 
         tick_length += (s64)(time_adjust * NSEC_PER_USEC / NTP_INTERVAL_FREQ)
                                                          << NTP_SCALE_SHIFT;
         time_adjust = 0;
+out:
+        return leap;
 }
 
 #ifdef CONFIG_GENERIC_CMOS_UPDATE
@@ -501,27 +501,6 @@ 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:
@@ -546,22 +525,6 @@ 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
@@ -643,9 +606,6 @@ 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) {
@@ -967,6 +927,4 @@ __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 5f458310668..c444da085a2 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -169,15 +169,6 @@ static struct timespec raw_time;
 /* flag for if timekeeping is suspended */
 int __read_mostly timekeeping_suspended;
 
-/* must hold xtime_lock */
-void timekeeping_leap_insert(int leapsecond)
-{
-        xtime.tv_sec += leapsecond;
-        wall_to_monotonic.tv_sec -= leapsecond;
-        update_vsyscall(&xtime, &wall_to_monotonic, timekeeper.clock,
-                        timekeeper.mult);
-}
-
 /**
  * timekeeping_forward_now - update clock to the current time
  *
@@ -828,9 +819,11 @@ 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;
                 xtime.tv_sec++;
-                second_overflow();
+                leap = second_overflow(xtime.tv_sec);
+                xtime.tv_sec += leap;
         }
 
         /* Accumulate raw time */
@@ -936,9 +929,12 @@ static void update_wall_time(void)
          * xtime.tv_nsec isn't larger then NSEC_PER_SEC
          */
         if (unlikely(xtime.tv_nsec >= NSEC_PER_SEC)) {
+                int leap;
                 xtime.tv_nsec -= NSEC_PER_SEC;
                 xtime.tv_sec++;
-                second_overflow();
+                leap = second_overflow(xtime.tv_sec);
+                xtime.tv_sec += leap;
+
         }
 
         /* check to see if there is a new clocksource to use */