[PATCH v4 09/13] rv: Prevent task migration while handling per-CPU events

[Gabriele Monaco <gmonaco@redhat.com>]

Tracepoint handlers are fully preemptible after a46023d5616 ("tracing:
Guard __DECLARE_TRACE() use of __DO_TRACE_CALL() with SRCU-fast"). When
a per-CPU monitor handles an event, it retrieves the monitor state using
a per-CPU pointer. If the event itself doesn't disable preemption, the
task can migrate to a different CPU and we risk updating the wrong
monitor.

Mitigate this by explicitly disabling task migration before acquiring
the monitor pointer. This cannot guarantee the monitor runs on the
correct CPU but reduces the race condition window and prevents warnings.

Reviewed-by: Wen Yang <wen.yang@linux.dev>
Reviewed-by: Nam Cao <namcao@linutronix.de>
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
---
 include/rv/da_monitor.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/include/rv/da_monitor.h b/include/rv/da_monitor.h
index 1f440c781..34b8fba9e 100644
--- a/include/rv/da_monitor.h
+++ b/include/rv/da_monitor.h
@@ -218,6 +218,10 @@ static inline void da_monitor_destroy(void)
 	da_monitor_sync_hook();
 }
 
+#ifndef da_implicit_guard
+#define da_implicit_guard()
+#endif
+
 #elif RV_MON_TYPE == RV_MON_PER_CPU
 /*
  * Functions to define, init and get a per-cpu monitor.
@@ -284,6 +288,10 @@ static inline void da_monitor_destroy(void)
 	da_monitor_sync_hook();
 }
 
+#ifndef da_implicit_guard
+#define da_implicit_guard() guard(migrate)()
+#endif
+
 #elif RV_MON_TYPE == RV_MON_PER_TASK
 /*
  * Functions to define, init and get a per-task monitor.
@@ -756,6 +764,7 @@ static inline bool __da_handle_start_run_event(struct da_monitor *da_mon,
  */
 static inline void da_handle_event(enum events event)
 {
+	da_implicit_guard();
 	__da_handle_event(da_get_monitor(), event, 0);
 }
 
@@ -771,6 +780,7 @@ static inline void da_handle_event(enum events event)
  */
 static inline bool da_handle_start_event(enum events event)
 {
+	da_implicit_guard();
 	return __da_handle_start_event(da_get_monitor(), event, 0);
 }
 
@@ -782,6 +792,7 @@ static inline bool da_handle_start_event(enum events event)
  */
 static inline bool da_handle_start_run_event(enum events event)
 {
+	da_implicit_guard();
 	return __da_handle_start_run_event(da_get_monitor(), event, 0);
 }
 
-- 
2.54.0