timekeeping: Provide y2038 safe accessor to the seconds portion of CLOCK_REALTIME

ktime_get_real_seconds() is the replacement function for get_seconds()
returning the seconds portion of CLOCK_REALTIME in a time64_t. For
64bit the function is equivivalent to get_seconds(), but for 32bit it
protects the readout with the timekeeper sequence count. This is
required because 32-bit machines cannot access 64-bit tk->xtime_sec
variable atomically.

[tglx: Massaged changelog and added docbook comment ]

Signed-off-by: Heena Sirwani <heenasirwani@gmail.com>
Reviewed-by: Arnd Bergman <arnd@arndb.de>
Cc: John Stultz <john.stultz@linaro.org>
Cc: opw-kernel@googlegroups.com
Link: http://lkml.kernel.org/r/7adcfaa8962b8ad58785d9a2456c3f77d93c0ffb.1414578445.git.heenasirwani@gmail.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

2 files changed, 31 insertions, 0 deletions

diff --git a/include/linux/timekeeping.h b/include/linux/timekeeping.h
index 115d55e11bc9..91454dea2bc6 100644
--- a/include/linux/timekeeping.h
+++ b/include/linux/timekeeping.h
@@ -29,6 +29,7 @@ struct timespec get_monotonic_coarse(void);
 extern void getrawmonotonic(struct timespec *ts);
 extern void ktime_get_ts64(struct timespec64 *ts);
 extern time64_t ktime_get_seconds(void);
+extern time64_t ktime_get_real_seconds(void);
 
 extern int __getnstimeofday64(struct timespec64 *tv);
 extern void getnstimeofday64(struct timespec64 *tv);
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index a693270efafb..0aef92a0a701 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -676,6 +676,36 @@ time64_t ktime_get_seconds(void)
 }
 EXPORT_SYMBOL_GPL(ktime_get_seconds);
 
+/**
+ * ktime_get_real_seconds - Get the seconds portion of CLOCK_REALTIME
+ *
+ * Returns the wall clock seconds since 1970. This replaces the
+ * get_seconds() interface which is not y2038 safe on 32bit systems.
+ *
+ * For 64bit systems the fast access to tk->xtime_sec is preserved. On
+ * 32bit systems the access must be protected with the sequence
+ * counter to provide "atomic" access to the 64bit tk->xtime_sec
+ * value.
+ */
+time64_t ktime_get_real_seconds(void)
+{
+        struct timekeeper *tk = &tk_core.timekeeper;
+        time64_t seconds;
+        unsigned int seq;
+
+        if (IS_ENABLED(CONFIG_64BIT))
+                return tk->xtime_sec;
+
+        do {
+                seq = read_seqcount_begin(&tk_core.seq);
+                seconds = tk->xtime_sec;
+
+        } while (read_seqcount_retry(&tk_core.seq, seq));
+
+        return seconds;
+}
+EXPORT_SYMBOL_GPL(ktime_get_real_seconds);
+
 #ifdef CONFIG_NTP_PPS
 
 /**