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Subject[tip:timers/urgent] posix-cpu-timers: Unbreak timer rearming
Commit-ID:  93ad0fc088c5b4631f796c995bdd27a082ef33a6
Author: Thomas Gleixner <>
AuthorDate: Fri, 11 Jan 2019 14:33:16 +0100
Committer: Thomas Gleixner <>
CommitDate: Tue, 15 Jan 2019 16:34:37 +0100

posix-cpu-timers: Unbreak timer rearming

The recent commit which prevented a division by 0 issue in the alarm timer
code broke posix CPU timers as an unwanted side effect.

The reason is that the common rearm code checks for timer->it_interval
being 0 now. What went unnoticed is that the posix cpu timer setup does not
initialize timer->it_interval as it stores the interval in CPU timer
specific storage. The reason for the separate storage is historical as the
posix CPU timers always had a 64bit nanoseconds representation internally
while timer->it_interval is type ktime_t which used to be a modified
timespec representation on 32bit machines.

Instead of reverting the offending commit and fixing the alarmtimer issue
in the alarmtimer code, store the interval in timer->it_interval at CPU
timer setup time so the common code check works. This also repairs the
existing inconistency of the posix CPU timer code which kept a single shot
timer armed despite of the interval being 0.

The separate storage can be removed in mainline, but that needs to be a
separate commit as the current one has to be backported to stable kernels.

Fixes: 0e334db6bb4b ("posix-timers: Fix division by zero bug")
Reported-by: H.J. Lu <>
Signed-off-by: Thomas Gleixner <>
Cc: John Stultz <>
Cc: Peter Zijlstra <>

kernel/time/posix-cpu-timers.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c
index 8f0644af40be..80f955210861 100644
--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -685,6 +685,7 @@ static int posix_cpu_timer_set(struct k_itimer *timer, int timer_flags,
* set up the signal and overrun bookkeeping.
timer->it.cpu.incr = timespec64_to_ns(&new->it_interval);
+ timer->it_interval = ns_to_ktime(timer->it.cpu.incr);

* This acts as a modification timestamp for the timer,
 \ /
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