clocksource: Avoid selecting mult values that might overflow when adjusted

For some frequencies, the clocks_calc_mult_shift() function will
unfortunately select mult values very close to 0xffffffff.  This
has the potential to overflow when NTP adjusts the clock, adding
to the mult value.

This patch adds a clocksource.maxadj value, which provides
an approximation of an 11% adjustment(NTP limits adjustments to
500ppm and the tick adjustment is limited to 10%), which could
be made to the clocksource.mult value. This is then used to both
check that the current mult value won't overflow/underflow, as
well as warning us if the timekeeping_adjust() code pushes over
that 11% boundary.

v2: Fix max_adjustment calculation, and improve WARN_ONCE
messages.

v3: Don't warn before maxadj has actually been set

CC: Yong Zhang <yong.zhang0@gmail.com>
CC: David Daney <ddaney.cavm@gmail.com>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Chen Jie <chenj@lemote.com>
CC: zhangfx <zhangfx@lemote.com>
CC: stable@kernel.org
Reported-by: Chen Jie <chenj@lemote.com>
Reported-by: zhangfx <zhangfx@lemote.com>
Tested-by: Yong Zhang <yong.zhang0@gmail.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>

1 files changed, 7 insertions, 0 deletions

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 2b021b0e8507..e65ff3171102 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -820,6 +820,13 @@ static void timekeeping_adjust(s64 offset)
         } else
                 return;
 
+        WARN_ONCE(timekeeper.clock->maxadj &&
+                        (timekeeper.mult + adj > timekeeper.clock->mult +
+                                                timekeeper.clock->maxadj),
+                        "Adjusting %s more then 11%% (%ld vs %ld)\n",
+                        timekeeper.clock->name, (long)timekeeper.mult + adj,
+                        (long)timekeeper.clock->mult +
+                                timekeeper.clock->maxadj);
         timekeeper.mult += adj;
         timekeeper.xtime_interval += interval;
         timekeeper.xtime_nsec -= offset;