[PATCH] use seq_lock for monotonic time

Monotonic clock code uses reader/writer lock which is prone to same
starvation problems as we saw with xtime.  This patch changes it to seq_lock
which is faster and won't starve writers in face of lots of readers.

arch/i386/kernel/timers/timer_tsc.c

@@ -39,7 +39,7 @@ static int delay_at_last_interrupt;
 static unsigned long last_tsc_low; /* lsb 32 bits of Time Stamp Counter */
 static unsigned long last_tsc_high; /* msb 32 bits of Time Stamp Counter */
 static unsigned long long monotonic_base;
-static rwlock_t monotonic_lock = RW_LOCK_UNLOCKED;
+static seqlock_t monotonic_lock = SEQLOCK_UNLOCKED;
 
 /* convert from cycles(64bits) => nanoseconds (64bits)
  *  basic equation:
@@ -111,12 +111,14 @@ static unsigned long get_offset_tsc(void)
 static unsigned long long monotonic_clock_tsc(void)
 {
 	unsigned long long last_offset, this_offset, base;
+	unsigned seq;
 	
 	/* atomically read monotonic base & last_offset */
-	read_lock_irq(&monotonic_lock);
-	last_offset = ((unsigned long long)last_tsc_high<<32)|last_tsc_low;
-	base = monotonic_base;
-	read_unlock_irq(&monotonic_lock);
+	do {
+		seq = read_seqbegin(&monotonic_lock);
+		last_offset = ((unsigned long long)last_tsc_high<<32)|last_tsc_low;
+		base = monotonic_base;
+	} while (read_seqretry(&monotonic_lock, seq));
 
 	/* Read the Time Stamp Counter */
 	rdtscll(this_offset);
@@ -135,7 +137,7 @@ static void mark_offset_tsc(void)
 	unsigned long long this_offset, last_offset;
 	static int lost_count = 0;
 	
-	write_lock(&monotonic_lock);
+	write_seqlock(&monotonic_lock);
 	last_offset = ((unsigned long long)last_tsc_high<<32)|last_tsc_low;
 	/*
 	 * It is important that these two operations happen almost at
@@ -204,7 +206,7 @@ static void mark_offset_tsc(void)
 	/* update the monotonic base value */
 	this_offset = ((unsigned long long)last_tsc_high<<32)|last_tsc_low;
 	monotonic_base += cycles_2_ns(this_offset - last_offset);
-	write_unlock(&monotonic_lock);
+	write_sequnlock(&monotonic_lock);
 
 	/* calculate delay_at_last_interrupt */
 	count = ((LATCH-1) - count) * TICK_SIZE;
@@ -236,7 +238,7 @@ static void mark_offset_tsc_hpet(void)
 	unsigned long long this_offset, last_offset;
  	unsigned long offset, temp, hpet_current;
 
-	write_lock(&monotonic_lock);
+	write_seqlock(&monotonic_lock);
 	last_offset = ((unsigned long long)last_tsc_high<<32)|last_tsc_low;
 	/*
 	 * It is important that these two operations happen almost at
@@ -264,7 +266,7 @@ static void mark_offset_tsc_hpet(void)
 	/* update the monotonic base value */
 	this_offset = ((unsigned long long)last_tsc_high<<32)|last_tsc_low;
 	monotonic_base += cycles_2_ns(this_offset - last_offset);
-	write_unlock(&monotonic_lock);
+	write_sequnlock(&monotonic_lock);
 
 	/* calculate delay_at_last_interrupt */
 	/*