[PATCH] oprofile: add SMP barriers for hrtimer hotplug code

[Will Deacon <will.deacon@arm.com>]

OProfile uses a CPU notifier to start and stop any hrtimers when CPUs change
between ONLINE and DEAD. A static int ctr_running is used to keep track of
the counter state.

This can lead to problems where writes to the state variable are re-ordered
with repect to reads of the variable occurring on other CPUs, meaning that
__oprofile_hrtimer_start may read ctr_running as 0 and not initialise the
hrtimer. Potential deadlock can occur in __oprofile_hrtimer_stop because
lock_hrtimer_base will poll until timer->base != NULL, which will never
happen.

This patch adds smp_mb()s to ensure that ctr_running mirrors the correct
counter state.

Cc: Santosh Shilimkar <santosh.shilimkar@ti.com>
Cc: Robert Richter <robert.richter@amd.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
---
 drivers/oprofile/timer_int.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/drivers/oprofile/timer_int.c b/drivers/oprofile/timer_int.c
index 0107251..38c1e1b 100644
--- a/drivers/oprofile/timer_int.c
+++ b/drivers/oprofile/timer_int.c
@@ -48,6 +48,7 @@ static int oprofile_hrtimer_start(void)
 {
 	get_online_cpus();
 	ctr_running = 1;
+	smp_mb();
 	on_each_cpu(__oprofile_hrtimer_start, NULL, 1);
 	put_online_cpus();
 	return 0;
@@ -70,6 +71,7 @@ static void oprofile_hrtimer_stop(void)
 	get_online_cpus();
 	for_each_online_cpu(cpu)
 		__oprofile_hrtimer_stop(cpu);
+	smp_mb();
 	ctr_running = 0;
 	put_online_cpus();
 }
-- 
1.7.0.4