timerfd: Provide timerfd_resume()

Resuming timekeeping is a clock-was-set event and uses the clock-was-set
notification mechanism. This is in the way of making the clock-was-set
update for hrtimers selective so unnecessary IPIs are avoided when a CPU
base does not have timers queued which are affected by the clock setting.

Provide a seperate timerfd_resume() interface so the resume logic and the
clock-was-set mechanism can be distangled in the core code.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210713135158.395287410@linutronix.de

fs/timerfd.c

@@ -115,6 +115,22 @@ void timerfd_clock_was_set(void)
 	rcu_read_unlock();
 }
 
+static void timerfd_resume_work(struct work_struct *work)
+{
+	timerfd_clock_was_set();
+}
+
+static DECLARE_WORK(timerfd_work, timerfd_resume_work);
+
+/*
+ * Invoked from timekeeping_resume(). Defer the actual update to work so
+ * timerfd_clock_was_set() runs in task context.
+ */
+void timerfd_resume(void)
+{
+	schedule_work(&timerfd_work);
+}
+
 static void __timerfd_remove_cancel(struct timerfd_ctx *ctx)
 {
 	if (ctx->might_cancel) {

include/linux/hrtimer.h

@@ -349,8 +349,10 @@ hrtimer_expires_remaining_adjusted(const struct hrtimer *timer)
 
 #ifdef CONFIG_TIMERFD
 extern void timerfd_clock_was_set(void);
+extern void timerfd_resume(void);
 #else
 static inline void timerfd_clock_was_set(void) { }
+static inline void timerfd_resume(void) { }
 #endif
 extern void hrtimers_resume(void);