Merge branch 'main' into zstd-linus

1 files changed, 42 insertions, 0 deletions

diff --git a/include/linux/preempt.h b/include/linux/preempt.h
index b4381f255a5c..0df425bf9bd7 100644
--- a/include/linux/preempt.h
+++ b/include/linux/preempt.h
@@ -421,4 +421,46 @@ static inline void migrate_enable(void) { }
 
 #endif /* CONFIG_SMP */
 
+/**
+ * preempt_disable_nested - Disable preemption inside a normally preempt disabled section
+ *
+ * Use for code which requires preemption protection inside a critical
+ * section which has preemption disabled implicitly on non-PREEMPT_RT
+ * enabled kernels, by e.g.:
+ *  - holding a spinlock/rwlock
+ *  - soft interrupt context
+ *  - regular interrupt handlers
+ *
+ * On PREEMPT_RT enabled kernels spinlock/rwlock held sections, soft
+ * interrupt context and regular interrupt handlers are preemptible and
+ * only prevent migration. preempt_disable_nested() ensures that preemption
+ * is disabled for cases which require CPU local serialization even on
+ * PREEMPT_RT. For non-PREEMPT_RT kernels this is a NOP.
+ *
+ * The use cases are code sequences which are not serialized by a
+ * particular lock instance, e.g.:
+ *  - seqcount write side critical sections where the seqcount is not
+ *    associated to a particular lock and therefore the automatic
+ *    protection mechanism does not work. This prevents a live lock
+ *    against a preempting high priority reader.
+ *  - RMW per CPU variable updates like vmstat.
+ */
+/* Macro to avoid header recursion hell vs. lockdep */
+#define preempt_disable_nested()				\
+do {								\
+	if (IS_ENABLED(CONFIG_PREEMPT_RT))			\
+		preempt_disable();				\
+	else							\
+		lockdep_assert_preemption_disabled();		\
+} while (0)
+
+/**
+ * preempt_enable_nested - Undo the effect of preempt_disable_nested()
+ */
+static __always_inline void preempt_enable_nested(void)
+{
+	if (IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_enable();
+}
+
 #endif /* __LINUX_PREEMPT_H */