[PATCH 1/2] [PRECISE] (pre-upstream) Fix clock_was_set so it is safe to call from atomic

[Brad Figg]

From: John Stultz <john.stultz at linaro.org>

BugLink: http://bugs.launchpad.net/bugs/1020285

Backport for 3.2.21

NOTE:This is a prerequisite patch that's required to
address the widely observed leap-second related futex/hrtimer
issues.

Currently clock_was_set() is unsafe to be called from atomic
context, as it calls on_each_cpu(). This causes problems when
we need to adjust the time from update_wall_time().

To fix this, introduce a work_struct so if we're in_atomic,
we can schedule work to do the necessary update after we're
out of the atomic section.

CC: Prarit Bhargava <prarit at redhat.com>
CC: stable at vger.kernel.org
CC: Thomas Gleixner <tglx at linutronix.de>
Reported-by: Jan Engelhardt <jengelh at inai.de>
Signed-off-by: John Stultz <johnstul at us.ibm.com>
Signed-off-by: Brad Figg <brad.figg at canonical.com>
---
 kernel/hrtimer.c |   16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
index ae34bf5..ee7a98d 100644
--- a/kernel/hrtimer.c
+++ b/kernel/hrtimer.c
@@ -746,7 +746,7 @@ static inline void retrigger_next_event(void *arg) { }
  * resolution timer interrupts. On UP we just disable interrupts and
  * call the high resolution interrupt code.
  */
-void clock_was_set(void)
+static void do_clock_was_set(struct work_struct *work)
 {
 #ifdef CONFIG_HIGH_RES_TIMERS
 	/* Retrigger the CPU local events everywhere */
@@ -754,6 +754,20 @@ void clock_was_set(void)
 #endif
 	timerfd_clock_was_set();
 }
+static DECLARE_WORK(clock_was_set_work, do_clock_was_set);
+
+void clock_was_set(void)
+{
+	/*
+	 * We can't call on_each_cpu() from atomic context,
+	 * so if we're in_atomic, schedule the clock_was_set
+	 * for later.
+	 */
+	if (in_atomic())
+		schedule_work(&clock_was_set_work);
+	else
+		do_clock_was_set(NULL);
+}
 
 /*
  * During resume we might have to reprogram the high resolution timer
-- 
1.7.9.5