timerfd: Allow timers to be cancelled when clock was set

Some applications must be aware of clock realtime being set
backward. A simple example is a clock applet which arms a timer for
the next minute display. If clock realtime is set backward then the
applet displays a stale time for the amount of time which the clock
was set backwards. Due to that applications poll the time because we
don't have an interface.

Extend the timerfd interface by adding a flag which puts the timer
onto a different internal realtime clock. All timers on this clock are
expired whenever the clock was set.

The timerfd core records the monotonic offset when the timer is
created. When the timer is armed, then the current offset is compared
to the previous recorded offset. When it has changed, then
timerfd_settime returns -ECANCELED. When a timer is read the offset is
compared and if it changed -ECANCELED returned to user space. Periodic
timers are not rearmed in the cancelation case.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: John Stultz <johnstul@us.ibm.com>
Cc: Chris Friesen <chris.friesen@genband.com>
Tested-by: Kay Sievers <kay.sievers@vrfy.org>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Davide Libenzi <davidel@xmailserver.org>
Reviewed-by: Alexander Shishkin <virtuoso@slind.org>
Link: http://lkml.kernel.org/r/%3Calpine.LFD.2.02.1104271359580.3323%40ionos%3E
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 6 insertions, 0 deletions

diff --git a/include/linux/time.h b/include/linux/time.h
index b3061782dec3..a9242773eb24 100644
--- a/include/linux/time.h
+++ b/include/linux/time.h
@@ -302,6 +302,12 @@ struct itimerval {
  * The IDs of various hardware clocks:
  */
 #define CLOCK_SGI_CYCLE                 10
+
+#ifdef __KERNEL__
+/* This clock is not exposed to user space */
+#define CLOCK_REALTIME_COS              15
+#endif
+
 #define MAX_CLOCKS                      16
 #define CLOCKS_MASK                     (CLOCK_REALTIME | CLOCK_MONOTONIC)
 #define CLOCKS_MONO                     CLOCK_MONOTONIC