[PATCH v2 02/10] sched: Use {READ,WRITE}_ONCE() for preempt_dynamic_mode

[Peter Zijlstra <peterz@infradead.org>]

Robots figured out you can read and write this concurrently and got
'upset'. Gemini even noted sched_dynamic_show() can generate
'confusing' output if it observed different values during the
printing.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/sched/core.c  |   15 ++++++++-------
 kernel/sched/debug.c |    5 +++--
 2 files changed, 11 insertions(+), 9 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7743,7 +7743,7 @@ static void __sched_dynamic_update(int m
 		break;
 	}
 
-	preempt_dynamic_mode = mode;
+	WRITE_ONCE(preempt_dynamic_mode, mode);
 }
 
 void sched_dynamic_update(int mode)
@@ -7784,12 +7784,13 @@ static void __init preempt_dynamic_init(
 	}
 }
 
-# define PREEMPT_MODEL_ACCESSOR(mode) \
-	bool preempt_model_##mode(void)						 \
-	{									 \
-		WARN_ON_ONCE(preempt_dynamic_mode == preempt_dynamic_undefined); \
-		return preempt_dynamic_mode == preempt_dynamic_##mode;		 \
-	}									 \
+# define PREEMPT_MODEL_ACCESSOR(mode)					\
+	bool preempt_model_##mode(void)					\
+	{								\
+		int mode = READ_ONCE(preempt_dynamic_mode);		\
+		WARN_ON_ONCE(mode == preempt_dynamic_undefined);	\
+		return mode == preempt_dynamic_##mode;			\
+	}								\
 	EXPORT_SYMBOL_GPL(preempt_model_##mode)
 
 PREEMPT_MODEL_ACCESSOR(none);
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -239,6 +239,7 @@ static ssize_t sched_dynamic_write(struc
 static int sched_dynamic_show(struct seq_file *m, void *v)
 {
 	int i = (IS_ENABLED(CONFIG_PREEMPT_RT) || IS_ENABLED(CONFIG_ARCH_HAS_PREEMPT_LAZY)) * 2;
+	int mode = READ_ONCE(preempt_dynamic_mode);
 	int j;
 
 	/* Count entries in NULL terminated preempt_modes */
@@ -247,10 +248,10 @@ static int sched_dynamic_show(struct seq
 	j -= !IS_ENABLED(CONFIG_ARCH_HAS_PREEMPT_LAZY);
 
 	for (; i < j; i++) {
-		if (preempt_dynamic_mode == i)
+		if (mode == i)
 			seq_puts(m, "(");
 		seq_puts(m, preempt_modes[i]);
-		if (preempt_dynamic_mode == i)
+		if (mode == i)
 			seq_puts(m, ")");
 
 		seq_puts(m, " ");