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

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Message-ID: <CAMe9rOrKc_i9ugz_EkA7zyQNOOVMfmtcv4=HM8JHFd+ixhprTg@mail.gmail.com>
Date:   Fri, 11 Jan 2019 08:10:33 -0800
From:   "H.J. Lu" <hjl.tools@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        John Stultz <john.stultz@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [patch 1/2] posix-cpu-timers: Unbreak timer rearming

On Fri, Jan 11, 2019 at 5:36 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> 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,
>
>

I verified that this patch works on 4.19.14 kernel.

Thanks.

-- 
H.J.