tracing: Fix wraparound problems in "uptime" trace clock

The "uptime" trace clock added in:

    commit 8aacf017b065a805d27467843490c976835eb4a5
    tracing: Add "uptime" trace clock that uses jiffies

has wraparound problems when the system has been up more
than 1 hour 11 minutes and 34 seconds. It converts jiffies
to nanoseconds using:
        (u64)jiffies_to_usecs(jiffy) * 1000ULL
but since jiffies_to_usecs() only returns a 32-bit value, it
truncates at 2^32 microseconds.  An additional problem on 32-bit
systems is that the argument is "unsigned long", so fixing the
return value only helps until 2^32 jiffies (49.7 days on a HZ=1000
system).

Avoid these problems by using jiffies_64 as our basis, and
not converting to nanoseconds (we do convert to clock_t because
user facing API must not be dependent on internal kernel
HZ values).

Link: http://lkml.kernel.org/p/99d63c5bfe9b320a3b428d773825a37095bf6a51.1405708254.git.tony.luck@intel.com

Cc: stable@vger.kernel.org # 3.10+
Fixes: 8aacf017b065 "tracing: Add "uptime" trace clock that uses jiffies"
Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

2 files changed, 6 insertions, 5 deletions

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index bda9621638cc..291397e66669 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -823,7 +823,7 @@ static struct {
         { trace_clock_local,    "local",        1 },
         { trace_clock_global,   "global",       1 },
         { trace_clock_counter,  "counter",      0 },
-        { trace_clock_jiffies,  "uptime",       1 },
+        { trace_clock_jiffies,  "uptime",       0 },
         { trace_clock,          "perf",         1 },
         ARCH_TRACE_CLOCKS
 };
diff --git a/kernel/trace/trace_clock.c b/kernel/trace/trace_clock.c
index 26dc348332b7..57b67b1f24d1 100644
--- a/kernel/trace/trace_clock.c
+++ b/kernel/trace/trace_clock.c
@@ -59,13 +59,14 @@ u64 notrace trace_clock(void)
 
 /*
  * trace_jiffy_clock(): Simply use jiffies as a clock counter.
+ * Note that this use of jiffies_64 is not completely safe on
+ * 32-bit systems. But the window is tiny, and the effect if
+ * we are affected is that we will have an obviously bogus
+ * timestamp on a trace event - i.e. not life threatening.
  */
 u64 notrace trace_clock_jiffies(void)
 {
-        u64 jiffy = jiffies - INITIAL_JIFFIES;
-
-        /* Return nsecs */
-        return (u64)jiffies_to_usecs(jiffy) * 1000ULL;
+        return jiffies_64_to_clock_t(jiffies_64 - INITIAL_JIFFIES);
 }
 
 /*