linux-kernel - [patch 1/2] posix-cpu-timers: Unbreak timer rearming

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Message-Id: <20190111133500.840117406@linutronix.de>
Date:   Fri, 11 Jan 2019 14:33:16 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     John Stultz <john.stultz@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "H.J. Lu" <hjl.tools@...il.com>
Subject: [patch 1/2] 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 <hjl.tools@...il.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Cc: stable@...r.kernel.org
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: John Stultz <john.stultz@...aro.org>
---
 kernel/time/posix-cpu-timers.c |    1 +
 1 file changed, 1 insertion(+)

--- 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_
 	 * 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,