timekeeping: Implement a shadow timekeeper

Use the shadow timekeeper to do the update_wall_time() adjustments and
then copy it over to the real timekeeper.

Keep the shadow timekeeper in sync when updating stuff outside of
update_wall_time().

This allows us to limit the timekeeper_seq hold time to the update of
the real timekeeper and the vsyscall data in the next patch.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>

1 files changed, 29 insertions, 12 deletions

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 38ac782c0ef8..d20ffdad62e8 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -29,6 +29,7 @@
 static struct timekeeper timekeeper;
 static DEFINE_RAW_SPINLOCK(timekeeper_lock);
 static seqcount_t timekeeper_seq;
+static struct timekeeper shadow_timekeeper;
 
 /* flag for if timekeeping is suspended */
 int __read_mostly timekeeping_suspended;
@@ -240,7 +241,7 @@ int pvclock_gtod_unregister_notifier(struct notifier_block *nb)
 EXPORT_SYMBOL_GPL(pvclock_gtod_unregister_notifier);
 
 /* must hold timekeeper_lock */
-static void timekeeping_update(struct timekeeper *tk, bool clearntp)
+static void timekeeping_update(struct timekeeper *tk, bool clearntp, bool mirror)
 {
         if (clearntp) {
                 tk->ntp_error = 0;
@@ -248,6 +249,9 @@ static void timekeeping_update(struct timekeeper *tk, bool clearntp)
         }
         update_vsyscall(tk);
         update_pvclock_gtod(tk);
+
+        if (mirror)
+                memcpy(&shadow_timekeeper, &timekeeper, sizeof(timekeeper));
 }
 
 /**
@@ -504,7 +508,7 @@ int do_settimeofday(const struct timespec *tv)
 
         tk_set_xtime(tk, tv);
 
-        timekeeping_update(tk, true);
+        timekeeping_update(tk, true, true);
 
         write_seqcount_end(&timekeeper_seq);
         raw_spin_unlock_irqrestore(&timekeeper_lock, flags);
@@ -548,7 +552,7 @@ int timekeeping_inject_offset(struct timespec *ts)
         tk_set_wall_to_mono(tk, timespec_sub(tk->wall_to_monotonic, *ts));
 
 error: /* even if we error out, we forwarded the time, so call update */
-        timekeeping_update(tk, true);
+        timekeeping_update(tk, true, true);
 
         write_seqcount_end(&timekeeper_seq);
         raw_spin_unlock_irqrestore(&timekeeper_lock, flags);
@@ -628,7 +632,7 @@ static int change_clocksource(void *data)
                 if (old->disable)
                         old->disable(old);
         }
-        timekeeping_update(tk, true);
+        timekeeping_update(tk, true, true);
 
         write_seqcount_end(&timekeeper_seq);
         raw_spin_unlock_irqrestore(&timekeeper_lock, flags);
@@ -809,6 +813,8 @@ void __init timekeeping_init(void)
         tmp.tv_nsec = 0;
         tk_set_sleep_time(tk, tmp);
 
+        memcpy(&shadow_timekeeper, &timekeeper, sizeof(timekeeper));
+
         write_seqcount_end(&timekeeper_seq);
         raw_spin_unlock_irqrestore(&timekeeper_lock, flags);
 }
@@ -865,7 +871,7 @@ void timekeeping_inject_sleeptime(struct timespec *delta)
 
         __timekeeping_inject_sleeptime(tk, delta);
 
-        timekeeping_update(tk, true);
+        timekeeping_update(tk, true, true);
 
         write_seqcount_end(&timekeeper_seq);
         raw_spin_unlock_irqrestore(&timekeeper_lock, flags);
@@ -947,7 +953,7 @@ static void timekeeping_resume(void)
         clock->cycle_last = cycle_now;
         tk->ntp_error = 0;
         timekeeping_suspended = 0;
-        timekeeping_update(tk, false);
+        timekeeping_update(tk, false, true);
         write_seqcount_end(&timekeeper_seq);
         raw_spin_unlock_irqrestore(&timekeeper_lock, flags);
 
@@ -1328,7 +1334,8 @@ static inline void old_vsyscall_fixup(struct timekeeper *tk)
 static void update_wall_time(void)
 {
         struct clocksource *clock;
-        struct timekeeper *tk = &timekeeper;
+        struct timekeeper *real_tk = &timekeeper;
+        struct timekeeper *tk = &shadow_timekeeper;
         cycle_t offset;
         int shift = 0, maxshift;
         unsigned long flags;
@@ -1340,16 +1347,16 @@ static void update_wall_time(void)
         if (unlikely(timekeeping_suspended))
                 goto out;
 
-        clock = tk->clock;
+        clock = real_tk->clock;
 
 #ifdef CONFIG_ARCH_USES_GETTIMEOFFSET
-        offset = tk->cycle_interval;
+        offset = real_tk->cycle_interval;
 #else
         offset = (clock->read(clock) - clock->cycle_last) & clock->mask;
 #endif
 
         /* Check if there's really nothing to do */
-        if (offset < tk->cycle_interval)
+        if (offset < real_tk->cycle_interval)
                 goto out;
 
         /*
@@ -1388,12 +1395,22 @@ static void update_wall_time(void)
 
         /* Update clock->cycle_last with the new value */
         clock->cycle_last = tk->cycle_last;
-        timekeeping_update(tk, false);
+        /*
+         * Update the real timekeeper.
+         *
+         * We could avoid this memcpy by switching pointers, but that
+         * requires changes to all other timekeeper usage sites as
+         * well, i.e. move the timekeeper pointer getter into the
+         * spinlocked/seqcount protected sections. And we trade this
+         * memcpy under the timekeeper_seq against one before we start
+         * updating.
+         */
+        memcpy(real_tk, tk, sizeof(*tk));
+        timekeeping_update(real_tk, false, false);
 
 out:
         write_seqcount_end(&timekeeper_seq);
         raw_spin_unlock_irqrestore(&timekeeper_lock, flags);
-
 }
 
 /**