Devi R K reported this issue, and went on to note: > We have written a program using real time clock and it has been raised to > the community. > > https://lore.kernel.org/lkml/alpine.DEB.2.21.1908191943280.1796@nanos.tec.linutronix.de/T/ [...] Thanks for pointing me at that thread. In particular, the test program at https://lore.kernel.org/lkml/alpine.DEB.2.21.1908191943280.1796@nanos.tec.linutronix.de/T/#m489d81abdfbb2699743e18c37657311f8d52a4cd [...] I think this patch does not really capture the details properly. The immediately preceding paragraph says: If the associated clock is either CLOCK_REALTIME or CLOCK_REALTIME_ALARM, the timer is absolute (TFD_TIMER_ABSTIME), and the flag TFD_TIMER_CANCEL_ON_SET was specified when calling timerfd_settime(), then read(2) fails with the error ECANCELED if the real-time clock undergoes a discontinuous change. (This allows the reading application to discover such discontinuous changes to the clock.) Following on from that, I think we should have a paragraph that says something like: If the associated clock is either CLOCK_REALTIME or CLOCK_REALTIME_ALARM, the timer is absolute (TFD_TIMER_ABSTIME), and the flag TFD_TIMER_CANCEL_ON_SET was not specified when calling timerfd_settime(), then a discontinuous negative change to the clock (e.g., clock_settime(2)) may cause read(2) to unblock, but return a value of 0 (i.e., no bytes read), if the clock change occurs after the time expired, but before the read(2) on the timerfd file descriptor. This seems consistent with Thomas's observations in https://lore.kernel.org/lkml/alpine.DEB.2.21.1908191943280.1796@nanos.tec.linutronix.de/T/#m49b78122b573a2749a05b720dc9fa036546db490 == Thomas Gleixner replied: Yes, that's correct. Accurate as always! This is pretty much in line with clock_nanosleep(CLOCK_REALTIME, TIMER_ABSTIME) which has a similar problem vs. observability in user space. clock_nanosleep(2) mutters: "POSIX.1 specifies that after changing the value of the CLOCK_REALTIME clock via clock_settime(2), the new clock value shall be used to determine the time at which a thread blocked on an absolute clock_nanosleep() will wake up; if the new clock value falls past the end of the sleep interval, then the clock_nanosleep() call will return immediately." which can be interpreted as guarantee that clock_nanosleep() never returns prematurely, i.e. the assert() in the below code would indicate a kernel failure: ret = clock_nanosleep(CLOCK_REALTIME, TIMER_ABSTIME, &expiry, NULL); if (!ret) { clock_gettime(CLOCK_REALTIME, &now); assert(now >= expiry); } But that assert can trigger when CLOCK_REALTIME was modified after the timer fired and the kernel decided to wake up the task and let it return to user space. clock_nanosleep(..., &expiry) arm_timer(expires); schedule(); -> timer interrupt now = ktime_get_real(); if (expires <= now) -------------------------------- After this point wakeup(); clock_settime(2) or adjtimex(2) which makes CLOCK_REALTIME jump back far enough will cause the above assert to trigger. ... return from syscall (retval == 0) There is no guarantee against clock_settime() coming after the wakeup. Even if we put another check into the return to user path then we won't catch a clock_settime() which comes right after that and before user space invokes clock_gettime(). POSIX spec Issue 7 (2018 edition) says: The suspension for the absolute clock_nanosleep() function (that is, with the TIMER_ABSTIME flag set) shall be in effect at least until the value of the corresponding clock reaches the absolute time specified by rqtp. And that's what the kernel implements for clock_nanosleep() and timerfd behaves exactly the same way. The wakeup of the waiter, i.e. task blocked in clock_nanosleep(2), read(2), poll(2), is not happening _before_ the absolute time specified is reached. If clock_settime() happens right before the expiry check, then it does the right thing, but any modification to the clock after the wakeup cannot be mitigated. At least not in a way which would make the assert() in the example code above a reliable indicator for a kernel fail. That's the reason why I rejected the attempt to mitigate that particular 0 tick issue in timerfd as it would just scratch a particular itch but still not provide any guarantee. So having the '0' return documented is the right way to go. Reviewed-by: Thomas Gleixner <tglx@linutronix.de> Reported-by: devi R.K <devi.feb27@gmail.com> Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>