[tip: sched/core] sched: Use {READ,WRITE}_ONCE() for preempt_dynamic_mode

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     77557002234546a4fb46ebf517d6cb1d515535a9
Gitweb:        https://git.kernel.org/tip/77557002234546a4fb46ebf517d6cb1d515535a9
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Mon, 11 May 2026 13:31:06 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Tue, 26 May 2026 13:53:13 +02:00

sched: Use {READ,WRITE}_ONCE() for preempt_dynamic_mode

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@xxxxxxxxxxxxx>
Link: https://patch.msgid.link/20260511120627.176946327@xxxxxxxxxxxxx
---
 kernel/sched/core.c  | 15 ++++++++-------
 kernel/sched/debug.c |  5 +++--
 2 files changed, 11 insertions(+), 9 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b3a416b..83202f0 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7979,7 +7979,7 @@ static void __sched_dynamic_update(int mode)
 		break;
 	}
 
-	preempt_dynamic_mode = mode;
+	WRITE_ONCE(preempt_dynamic_mode, mode);
 }
 
 void sched_dynamic_update(int mode)
@@ -8020,12 +8020,13 @@ static void __init preempt_dynamic_init(void)
 	}
 }
 
-# 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);
diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index af13a89..4cdf2f9 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -281,6 +281,7 @@ static ssize_t sched_dynamic_write(struct file *filp, const char __user *ubuf,
 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 */
@@ -289,10 +290,10 @@ static int sched_dynamic_show(struct seq_file *m, void *v)
 	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, " ");