[PATCH v3] locking/lock_events: Use this_cpu_add() when necessary

[Waiman Long <longman@redhat.com>]

The kernel test robot has reported that the use of __this_cpu_add()
causes bug messages like:

  BUG: using __this_cpu_add() in preemptible [00000000] code: ...

This is only an issue on preempt kernel where preemption can happen in
the middle of a percpu operation. We are still using __this_cpu_*() for
!preempt kernel to avoid additional overhead in case CONFIG_PREEMPT_COUNT
is set.

 v2: Simplify the condition to just preempt or !preempt.
 v3: Document the imprecise nature of the percpu count.

Fixes: a8654596f0371 ("locking/rwsem: Enable lock event counting")
Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/locking/lock_events.h | 32 ++++++++++++++++++++++++++++++--
 1 file changed, 30 insertions(+), 2 deletions(-)

diff --git a/kernel/locking/lock_events.h b/kernel/locking/lock_events.h
index feb1acc54611..6bcb96b7e1d3 100644
--- a/kernel/locking/lock_events.h
+++ b/kernel/locking/lock_events.h
@@ -30,13 +30,41 @@ enum lock_events {
  */
 DECLARE_PER_CPU(unsigned long, lockevents[lockevent_num]);
 
+/*
+ * The purpose of the lock event counting subsystem is to provide a low
+ * overhead way to record the number of specific locking events by using
+ * percpu counters. It is the percpu sum that matters, not specifically
+ * how many of them happens in each cpu.
+ *
+ * It is possible that the same percpu counter may be modified in both
+ * the process and interrupt contexts. For architectures that perform
+ * percpu operation with multiple instructions, it is possible to lose
+ * count if a process context percpu update is interrupted in the middle
+ * and the same counter is updated in the interrupt context. Therefore,
+ * the generated percpu sum may not be precise. The error, if any, should
+ * be small and insignificant.
+ *
+ * In !preempt kernel, we can just use __this_cpu_*() as preemption
+ * won't happen in the middle of the percpu operation. In preempt kernel,
+ * preemption happens in the middle of the percpu operation may produce
+ * incorrect result that can be signficant if the task is moved to a
+ * different cpu.
+ */
+#ifdef CONFIG_PREEMPT
+#define lockevent_percpu_inc(x)		this_cpu_inc(x)
+#define lockevent_percpu_add(x, v)	this_cpu_add(x, v)
+#else
+#define lockevent_percpu_inc(x)		__this_cpu_inc(x)
+#define lockevent_percpu_add(x, v)	__this_cpu_add(x, v)
+#endif
+
 /*
  * Increment the PV qspinlock statistical counters
  */
 static inline void __lockevent_inc(enum lock_events event, bool cond)
 {
 	if (cond)
-		__this_cpu_inc(lockevents[event]);
+		lockevent_percpu_inc(lockevents[event]);
 }
 
 #define lockevent_inc(ev)	  __lockevent_inc(LOCKEVENT_ ##ev, true)
@@ -44,7 +72,7 @@ static inline void __lockevent_inc(enum lock_events event, bool cond)
 
 static inline void __lockevent_add(enum lock_events event, int inc)
 {
-	__this_cpu_add(lockevents[event], inc);
+	lockevent_percpu_add(lockevents[event], inc);
 }
 
 #define lockevent_add(ev, c)	__lockevent_add(LOCKEVENT_ ##ev, c)
-- 
2.18.1