[PATCH 2/2] timers: introduce timer_try_add_on_cpu.

[Imran Khan <imran.f.khan@oracle.com>]

Timers started using add_timer_on, can get stuck if the
specified cpu is offline. If the user of add_timer_on
can't guarantee that the specified cpu is online and
ends up starting timer on an offline cpu, then that
timer may not give expected results.
Such users can use new interface timer_try_add_on_cpu,
which starts timer on a given cpu, only after ensuring
that it remains online. If it sees that the specified cpu
is offline or if it can't ensure that the cpu is online,
it does not start timer on any cpu and leaves the decision
of selecting other cpu to the caller.
Thus it ensures that started timer does not get lost, because
it was started on an offlined cpu.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Imran Khan <imran.f.khan@oracle.com>
---
 include/linux/timer.h |  1 +
 kernel/time/timer.c   | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/include/linux/timer.h b/include/linux/timer.h
index e67ecd1cbc97d..210c15527b325 100644
--- a/include/linux/timer.h
+++ b/include/linux/timer.h
@@ -148,6 +148,7 @@ static inline int timer_pending(const struct timer_list * timer)
 }
 
 extern void add_timer_on(struct timer_list *timer, int cpu);
+extern bool timer_try_add_on_cpu(struct timer_list *timer, int cpu);
 extern int mod_timer(struct timer_list *timer, unsigned long expires);
 extern int mod_timer_pending(struct timer_list *timer, unsigned long expires);
 extern int timer_reduce(struct timer_list *timer, unsigned long expires);
diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index ec9eb58e45241..800ed9b4dea7a 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1394,6 +1394,39 @@ void add_timer_on(struct timer_list *timer, int cpu)
 }
 EXPORT_SYMBOL_GPL(add_timer_on);
 
+/**
+ * timer_try_add_on_cpu - Try to start a timer on a particular CPU,
+ *			  after ensuring that it is and remains online.
+ * @timer:	The timer to be started
+ * @cpu:	The CPU to start it on
+ *
+ * Check and ensure that specified cpu is around, before starting a timer
+ * on it.
+ *
+ * Return:
+ * * %true  - If timer was started on an online cpu
+ * * %false - If the specified cpu was offline or if its online status
+ *	      could not be ensured due to unavailability of hotplug lock.
+ */
+bool timer_try_add_on_cpu(struct timer_list *timer, int cpu)
+{
+	bool ret = true;
+
+	if (unlikely(!cpu_online(cpu)))
+		ret = false;
+	else if (cpus_read_trylock()) {
+		if (likely(cpu_online(cpu)))
+			add_timer_on(timer, cpu);
+		else
+			ret = false;
+		cpus_read_unlock();
+	} else
+		ret = false;
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(timer_try_add_on_cpu);
+
 /**
  * __timer_delete - Internal function: Deactivate a timer
  * @timer:	The timer to be deactivated
-- 
2.34.1