[3.13.y.z extended stable] Patch "tracing: Fix wraparound problems in "uptime" trace clock" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    tracing: Fix wraparound problems in "uptime" trace clock

to the linux-3.13.y-queue branch of the 3.13.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.13.y-queue

This patch is scheduled to be released in version 3.13.11.6.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.13.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From bed00dd3d4d716a52b9f4f62901c191c994a5846 Mon Sep 17 00:00:00 2001
From: Tony Luck <tony.luck at intel.com>
Date: Fri, 18 Jul 2014 11:43:01 -0700
Subject: tracing: Fix wraparound problems in "uptime" trace clock

commit 58d4e21e50ff3cc57910a8abc20d7e14375d2f61 upstream.

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

Fixes: 8aacf017b065 "tracing: Add "uptime" trace clock that uses jiffies"
Signed-off-by: Tony Luck <tony.luck at intel.com>
Signed-off-by: Steven Rostedt <rostedt at goodmis.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 kernel/trace/trace.c       | 2 +-
 kernel/trace/trace_clock.c | 9 +++++----
 2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 83cb797..9270fbc 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -784,7 +784,7 @@ 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 26dc348..57b67b1 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);
 }

 /*
--
1.9.1