[RFC][PATCH 6/7] context_tracking: Provide SMP ordering using RCU

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

Use rcu_user_{enter,exit}() calls to provide SMP ordering on context
tracking state stores:

__context_tracking_exit()
  __this_cpu_write(context_tracking.state, CONTEXT_KERNEL)
  rcu_user_exit()
    rcu_eqs_exit()
      rcu_dynticks_eqs_eit()
        rcu_dynticks_inc()
          atomic_add_return() /* smp_mb */

__context_tracking_enter()
  rcu_user_enter()
    rcu_eqs_enter()
      rcu_dynticks_eqs_enter()
        rcu_dynticks_inc()
	  atomic_add_return() /* smp_mb */
  __this_cpu_write(context_tracking.state, state)

This separates USER/KERNEL state with an smp_mb() on each side,
therefore, a user of context_tracking_state_cpu() can say the CPU must
pass through an smp_mb() before changing.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 include/linux/context_tracking_state.h |   12 ++++++++++++
 kernel/context_tracking.c              |    7 ++++---
 2 files changed, 16 insertions(+), 3 deletions(-)

--- a/include/linux/context_tracking_state.h
+++ b/include/linux/context_tracking_state.h
@@ -45,11 +45,23 @@ static __always_inline bool context_trac
 {
 	return __this_cpu_read(context_tracking.state) == CONTEXT_USER;
 }
+
+static __always_inline bool context_tracking_state_cpu(int cpu)
+{
+	struct context_tracking *ct = per_cpu_ptr(&context_tracking);
+
+	if (!context_tracking_enabled() || !ct->active)
+		return CONTEXT_DISABLED;
+
+	return ct->state;
+}
+
 #else
 static inline bool context_tracking_in_user(void) { return false; }
 static inline bool context_tracking_enabled(void) { return false; }
 static inline bool context_tracking_enabled_cpu(int cpu) { return false; }
 static inline bool context_tracking_enabled_this_cpu(void) { return false; }
+static inline bool context_tracking_state_cpu(int cpu) { return CONTEXT_DISABLED; }
 #endif /* CONFIG_CONTEXT_TRACKING */
 
 #endif
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c
@@ -82,7 +82,7 @@ void noinstr __context_tracking_enter(en
 				vtime_user_enter(current);
 				instrumentation_end();
 			}
-			rcu_user_enter();
+			rcu_user_enter(); /* smp_mb */
 		}
 		/*
 		 * Even if context tracking is disabled on this CPU, because it's outside
@@ -149,12 +149,14 @@ void noinstr __context_tracking_exit(enu
 		return;
 
 	if (__this_cpu_read(context_tracking.state) == state) {
+		__this_cpu_write(context_tracking.state, CONTEXT_KERNEL);
+
 		if (__this_cpu_read(context_tracking.active)) {
 			/*
 			 * We are going to run code that may use RCU. Inform
 			 * RCU core about that (ie: we may need the tick again).
 			 */
-			rcu_user_exit();
+			rcu_user_exit(); /* smp_mb */
 			if (state == CONTEXT_USER) {
 				instrumentation_begin();
 				vtime_user_exit(current);
@@ -162,7 +164,6 @@ void noinstr __context_tracking_exit(enu
 				instrumentation_end();
 			}
 		}
-		__this_cpu_write(context_tracking.state, CONTEXT_KERNEL);
 	}
 	context_tracking_recursion_exit();
 }