[PATCH] Add getnstimestamp function

There are several functions that might seem appropriate for a timestamp:

get_cycles()
current_kernel_time()
do_gettimeofday()
<read jiffies/jiffies_64>

Each has problems with combinations of SMP-safety, low resolution, and
monotonicity. This patch adds a new function that returns a monotonic SMP-safe
timestamp with nanosecond resolution where available.

Changes:
	Split timestamp into separate patch
	Moved to kernel/time.c
	Renamed to getnstimestamp
	Fixed unintended-pointer-arithmetic bug
Signed-off-by: Matt Helsley <matthltc@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/time.h

@@ -95,6 +95,7 @@ struct itimerval;
 extern int do_setitimer(int which, struct itimerval *value, struct itimerval *ovalue);
 extern int do_getitimer(int which, struct itimerval *value);
 extern void getnstimeofday (struct timespec *tv);
+extern void getnstimestamp(struct timespec *ts);
 
 extern struct timespec timespec_trunc(struct timespec t, unsigned gran);
 

kernel/time.c

@@ -561,6 +561,28 @@ void getnstimeofday(struct timespec *tv)
 EXPORT_SYMBOL_GPL(getnstimeofday);
 #endif
 
+void getnstimestamp(struct timespec *ts)
+{
+	unsigned int seq;
+	struct timespec wall2mono;
+
+	/* synchronize with settimeofday() changes */
+	do {
+		seq = read_seqbegin(&xtime_lock);
+		getnstimeofday(ts);
+		wall2mono = wall_to_monotonic;
+	} while(unlikely(read_seqretry(&xtime_lock, seq)));
+
+	/* adjust to monotonicaly-increasing values */
+	ts->tv_sec += wall2mono.tv_sec;
+	ts->tv_nsec += wall2mono.tv_nsec;
+	while (unlikely(ts->tv_nsec >= NSEC_PER_SEC)) {
+		ts->tv_nsec -= NSEC_PER_SEC;
+		ts->tv_sec++;
+	}
+}
+EXPORT_SYMBOL_GPL(getnstimestamp);
+
 #if (BITS_PER_LONG < 64)
 u64 get_jiffies_64(void)
 {