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    Subject[PATCH 3.12 84/94] tracing: Fix wraparound problems in "uptime" trace clock
    Date
    From: Tony Luck <tony.luck@intel.com>

    3.12-stable review patch. If anyone has any objections, please let me know.

    ===============

    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@intel.com>
    Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
    Signed-off-by: Jiri Slaby <jslaby@suse.cz>
    ---
    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 b7566fe4d607..dcdf4e682dd4 100644
    --- a/kernel/trace/trace.c
    +++ b/kernel/trace/trace.c
    @@ -764,7 +764,7 @@ 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);
    }

    /*
    --
    2.0.1


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