[RFC PATCH v2 01/10] rv/da: fix monitor start ordering and memory ordering for monitoring flag

[wen.yang@linux.dev]

From: Wen Yang <wen.yang@linux.dev>

da_monitor_start() set monitoring=1 before calling da_monitor_init_hook(),
may racing with the sched_switch handler:

  da_monitor_start()               sched_switch handler
  -------------------------        ---------------------------------
  da_mon->monitoring = 1;
                                   if (da_monitoring(da_mon))  /* true  */
                                       ha_start_timer_ns(...);
                                       /* hrtimer->base == NULL, crash */
  da_monitor_init_hook(da_mon);
  /* hrtimer_setup() sets base */

Fix the ordering and pair with release/acquire semantics:

  da_monitor_init_hook(da_mon);
  smp_store_release(&da_mon->monitoring, 1);    /* da_monitor_start()  */
  return smp_load_acquire(&da_mon->monitoring); /* da_monitoring()     */

On ARM64 a plain STR + LDR does not form a release-acquire pair, so
the load can observe monitoring=1 while hrtimer->base is still NULL.
The plain accesses are also data races under KCSAN.

Use WRITE_ONCE for the monitoring=0 store in da_monitor_reset() to
cover the reset path.

Fixes: 792575348ff7 ("rv/include: Add deterministic automata monitor definition via C macros")
Signed-off-by: Wen Yang <wen.yang@linux.dev>
---
 include/rv/da_monitor.h | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/include/rv/da_monitor.h b/include/rv/da_monitor.h
index 39765ff6f098..00ded3d5ab3f 100644
--- a/include/rv/da_monitor.h
+++ b/include/rv/da_monitor.h
@@ -82,7 +82,7 @@ static void react(enum states curr_state, enum events event)
 static inline void da_monitor_reset(struct da_monitor *da_mon)
 {
 	da_monitor_reset_hook(da_mon);
-	da_mon->monitoring = 0;
+	WRITE_ONCE(da_mon->monitoring, 0);
 	da_mon->curr_state = model_get_initial_state();
 }
 
@@ -95,8 +95,9 @@ static inline void da_monitor_reset(struct da_monitor *da_mon)
 static inline void da_monitor_start(struct da_monitor *da_mon)
 {
 	da_mon->curr_state = model_get_initial_state();
-	da_mon->monitoring = 1;
 	da_monitor_init_hook(da_mon);
+	/* Pairs with smp_load_acquire in da_monitoring(). */
+	smp_store_release(&da_mon->monitoring, 1);
 }
 
 /*
@@ -104,7 +105,8 @@ static inline void da_monitor_start(struct da_monitor *da_mon)
  */
 static inline bool da_monitoring(struct da_monitor *da_mon)
 {
-	return da_mon->monitoring;
+	/* Pairs with smp_store_release in da_monitor_start(). */
+	return smp_load_acquire(&da_mon->monitoring);
 }
 
 /*
-- 
2.25.1