Messages in this thread Patch in this message | | | From | John Stultz <> | Subject | [PATCH 1/2] [RFC] time: Fix problem with large timespecs & ktime_get_update_offsets | Date | Tue, 31 Jul 2012 02:35:47 -0400 |
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There's currently a slight difference in ktime_get_update_offsets() vs ktime_get() which can result in boot time crashes when booting with insane CMOS clock values larger then ~2264.
ktime_get() does basically the following: return timespec_to_ktime(timespec_add(xtime, wall_to_monotonic))
Where as ktime_get_update_offsets does approximately: return ktime_sub(timespec_to_ktime(xtime), realtime_offset);
The problem is, at boot we set xtime = year 8200 and wall_to_monotonic = year -8200, ktime_get adds both values, mostly nulling the difference out (leaving only how long the system has been up), then converts that relatively small value to a ktime_t properly without losing any information.
ktime_get_update_offsets however, since it converts xtime (again set to some value greater then year 8200), to a ktime, it gets clamped at KTIME_MAX, then we subtract realtime_offset, which is _also_ clamped at KTIME_MAX, resulting in us always returning almost[1] zero. This causes us to stop expiring timers.
Now, one of the reasons Thomas and I changed the logic was that using the precalculated realtime_offset was slightly more efficient then re-adding xtime and wall_to_monotonic's components separately. But how valuable this unmeasured slight efficiency is vs extra robustness for crazy time values is questionable.
So switch back to the ktime_get implementation for ktime_get_update_offsets
Cc: Ingo Molnar <mingo@kernel.org> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Prarit Bhargava <prarit@redhat.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Zhouping Liu <zliu@redhat.com> Cc: CAI Qian <caiqian@redhat.com> Reported-by: CAI Qian <caiqian@redhat.com> Signed-off-by: John Stultz <john.stultz@linaro.org> --- kernel/time/timekeeping.c | 12 +++++------- 1 file changed, 5 insertions(+), 7 deletions(-)
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c index 3447cfa..96179ab 100644 --- a/kernel/time/timekeeping.c +++ b/kernel/time/timekeeping.c @@ -1283,15 +1283,15 @@ void get_xtime_and_monotonic_and_sleep_offset(struct timespec *xtim, */ ktime_t ktime_get_update_offsets(ktime_t *offs_real, ktime_t *offs_boot) { - ktime_t now; unsigned int seq; u64 secs, nsecs; do { seq = read_seqbegin(&timekeeper.lock); - - secs = timekeeper.xtime.tv_sec; - nsecs = timekeeper.xtime.tv_nsec; + secs = timekeeper.xtime.tv_sec + + timekeeper.wall_to_monotonic.tv_sec; + nsecs = timekeeper.xtime.tv_nsec + + timekeeper.wall_to_monotonic.tv_nsec; nsecs += timekeeping_get_ns(); /* If arch requires, add in gettimeoffset() */ nsecs += arch_gettimeoffset(); @@ -1300,9 +1300,7 @@ ktime_t ktime_get_update_offsets(ktime_t *offs_real, ktime_t *offs_boot) *offs_boot = timekeeper.offs_boot; } while (read_seqretry(&timekeeper.lock, seq)); - now = ktime_add_ns(ktime_set(secs, 0), nsecs); - now = ktime_sub(now, *offs_real); - return now; + return ktime_add_ns(ktime_set(secs, 0), nsecs); } #endif -- 1.7.9.5
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