linux-kernel - Re: timer_settime() and ECANCELED

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Message-ID: <87bloanh89.fsf@nanos.tec.linutronix.de>
Date:   Thu, 02 Apr 2020 15:35:02 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Michael Kerrisk \(man-pages\)" <mtk.manpages@...il.com>
Cc:     mtk.manpages@...il.com, linux-man <linux-man@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>, arul.jeniston@...il.com,
        "devi R.K" <devi.feb27@...il.com>,
        Marc Lehmann <debian-reportbug@...n9.de>,
        John Stultz <john.stultz@...aro.org>,
        Andrei Vagin <avagin@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>
Subject: Re: timer_settime() and ECANCELED

"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com> writes:
> NOTES
>        Suppose  the  following scenario for CLOCK_REALTIME or CLOCK_REAL‐
>        TIME_ALARM timer that was created with timerfd_create():
>
>        (a) The  timer  has  been  started  (timerfd_settime())  with  the
>            TFD_TIMER_ABSTIME and TFD_TIMER_CANCEL_ON_SET flags;
>
>        (b) A discontinuous change (e.g.  settimeofday(2)) is subsequently
>            made to the CLOCK_REALTIME clock; and
>
>        (c) the caller once more  calls  timerfd_settime()  to  rearm  the
>            timer (without first doing a read(2) on the file descriptor).
>
>        In this case the following occurs:
>
>        · The  timerfd_settime()  returns  -1 with errno set to ECANCELED.
>          (This enables the caller to know that  the  previous  timer  was
>          affected by a discontinuous change to the clock.)
>
>        · The  timer is successfully rearmed with the settings provided in
>          the second timerfd_settime() call.  (This was probably an imple‐
>          mentation  accident,  but  won't be fixed now, in case there are
>          applications that depend on this behaviour.)

Clear enough.

Thanks Michael!