Subject: timekeeping: Split jiffies seqlock
From: Thomas Gleixner <tglx@linutronix.de>
Date: Thu, 14 Feb 2013 22:36:59 +0100

Replace jiffies_lock seqlock with a simple seqcounter and a rawlock so
it can be taken in atomic context on RT.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 kernel/time/jiffies.c     |    7 ++++---
 kernel/time/tick-common.c |   10 ++++++----
 kernel/time/tick-sched.c  |   19 ++++++++++++-------
 kernel/time/timekeeping.c |    6 ++++--
 kernel/time/timekeeping.h |    3 ++-
 5 files changed, 28 insertions(+), 17 deletions(-)

--- a/kernel/time/jiffies.c
+++ b/kernel/time/jiffies.c
@@ -74,7 +74,8 @@ static struct clocksource clocksource_ji
 	.max_cycles	= 10,
 };
 
-__cacheline_aligned_in_smp DEFINE_SEQLOCK(jiffies_lock);
+__cacheline_aligned_in_smp DEFINE_RAW_SPINLOCK(jiffies_lock);
+__cacheline_aligned_in_smp seqcount_t jiffies_seq;
 
 #if (BITS_PER_LONG < 64)
 u64 get_jiffies_64(void)
@@ -83,9 +84,9 @@ u64 get_jiffies_64(void)
 	u64 ret;
 
 	do {
-		seq = read_seqbegin(&jiffies_lock);
+		seq = read_seqcount_begin(&jiffies_seq);
 		ret = jiffies_64;
-	} while (read_seqretry(&jiffies_lock, seq));
+	} while (read_seqcount_retry(&jiffies_seq, seq));
 	return ret;
 }
 EXPORT_SYMBOL(get_jiffies_64);
--- a/kernel/time/tick-common.c
+++ b/kernel/time/tick-common.c
@@ -78,13 +78,15 @@ int tick_is_oneshot_available(void)
 static void tick_periodic(int cpu)
 {
 	if (tick_do_timer_cpu == cpu) {
-		write_seqlock(&jiffies_lock);
+		raw_spin_lock(&jiffies_lock);
+		write_seqcount_begin(&jiffies_seq);
 
 		/* Keep track of the next tick event */
 		tick_next_period = ktime_add(tick_next_period, tick_period);
 
 		do_timer(1);
-		write_sequnlock(&jiffies_lock);
+		write_seqcount_end(&jiffies_seq);
+		raw_spin_unlock(&jiffies_lock);
 		update_wall_time();
 	}
 
@@ -146,9 +148,9 @@ void tick_setup_periodic(struct clock_ev
 		ktime_t next;
 
 		do {
-			seq = read_seqbegin(&jiffies_lock);
+			seq = read_seqcount_begin(&jiffies_seq);
 			next = tick_next_period;
-		} while (read_seqretry(&jiffies_lock, seq));
+		} while (read_seqcount_retry(&jiffies_seq, seq));
 
 		clockevents_set_state(dev, CLOCK_EVT_STATE_ONESHOT);
 
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -62,7 +62,8 @@ static void tick_do_update_jiffies64(kti
 		return;
 
 	/* Reevalute with jiffies_lock held */
-	write_seqlock(&jiffies_lock);
+	raw_spin_lock(&jiffies_lock);
+	write_seqcount_begin(&jiffies_seq);
 
 	delta = ktime_sub(now, last_jiffies_update);
 	if (delta.tv64 >= tick_period.tv64) {
@@ -85,10 +86,12 @@ static void tick_do_update_jiffies64(kti
 		/* Keep the tick_next_period variable up to date */
 		tick_next_period = ktime_add(last_jiffies_update, tick_period);
 	} else {
-		write_sequnlock(&jiffies_lock);
+		write_seqcount_end(&jiffies_seq);
+		raw_spin_unlock(&jiffies_lock);
 		return;
 	}
-	write_sequnlock(&jiffies_lock);
+	write_seqcount_end(&jiffies_seq);
+	raw_spin_unlock(&jiffies_lock);
 	update_wall_time();
 }
 
@@ -99,12 +102,14 @@ static ktime_t tick_init_jiffy_update(vo
 {
 	ktime_t period;
 
-	write_seqlock(&jiffies_lock);
+	raw_spin_lock(&jiffies_lock);
+	write_seqcount_begin(&jiffies_seq);
 	/* Did we start the jiffies update yet ? */
 	if (last_jiffies_update.tv64 == 0)
 		last_jiffies_update = tick_next_period;
 	period = last_jiffies_update;
-	write_sequnlock(&jiffies_lock);
+	write_seqcount_end(&jiffies_seq);
+	raw_spin_unlock(&jiffies_lock);
 	return period;
 }
 
@@ -578,10 +583,10 @@ static ktime_t tick_nohz_stop_sched_tick
 
 	/* Read jiffies and the time when jiffies were updated last */
 	do {
-		seq = read_seqbegin(&jiffies_lock);
+		seq = read_seqcount_begin(&jiffies_seq);
 		last_update = last_jiffies_update;
 		last_jiffies = jiffies;
-	} while (read_seqretry(&jiffies_lock, seq));
+	} while (read_seqcount_retry(&jiffies_seq, seq));
 
 	if (rcu_needs_cpu(&rcu_delta_jiffies) ||
 	    arch_needs_cpu() || irq_work_needs_cpu()) {
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -2065,8 +2065,10 @@ EXPORT_SYMBOL(hardpps);
  */
 void xtime_update(unsigned long ticks)
 {
-	write_seqlock(&jiffies_lock);
+	raw_spin_lock(&jiffies_lock);
+	write_seqcount_begin(&jiffies_seq);
 	do_timer(ticks);
-	write_sequnlock(&jiffies_lock);
+	write_seqcount_end(&jiffies_seq);
+	raw_spin_unlock(&jiffies_lock);
 	update_wall_time();
 }
--- a/kernel/time/timekeeping.h
+++ b/kernel/time/timekeeping.h
@@ -22,7 +22,8 @@ extern void timekeeping_resume(void);
 extern void do_timer(unsigned long ticks);
 extern void update_wall_time(void);
 
-extern seqlock_t jiffies_lock;
+extern raw_spinlock_t jiffies_lock;
+extern seqcount_t jiffies_seq;
 
 #define CS_NAME_LEN	32