Subject: u64_stats: Disable preemption on 32bit-UP/SMP with RT during updates
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Mon Aug 17 12:28:10 2020 +0200

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

On RT the seqcount_t is required even on UP because the softirq can be
preempted. The IRQ handler is threaded so it is also preemptible.

Disable preemption on 32bit-RT during value updates. There is no need to
disable interrupts on RT because the handler is run threaded. Therefore
disabling preemption is enough to guarantee that the update is not
interruped.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>


---
 include/linux/u64_stats_sync.h |   42 +++++++++++++++++++++++++++--------------
 1 file changed, 28 insertions(+), 14 deletions(-)
---
--- a/include/linux/u64_stats_sync.h
+++ b/include/linux/u64_stats_sync.h
@@ -66,7 +66,7 @@
 #include <linux/seqlock.h>
 
 struct u64_stats_sync {
-#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
+#if BITS_PER_LONG==32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
 	seqcount_t	seq;
 #endif
 };
@@ -125,7 +125,7 @@ static inline void u64_stats_inc(u64_sta
 }
 #endif
 
-#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
+#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
 #define u64_stats_init(syncp)	seqcount_init(&(syncp)->seq)
 #else
 static inline void u64_stats_init(struct u64_stats_sync *syncp)
@@ -135,15 +135,19 @@ static inline void u64_stats_init(struct
 
 static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
 {
-#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
+#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
+	if (IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_disable();
 	write_seqcount_begin(&syncp->seq);
 #endif
 }
 
 static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
 {
-#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
+#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
 	write_seqcount_end(&syncp->seq);
+	if (IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_enable();
 #endif
 }
 
@@ -152,8 +156,11 @@ u64_stats_update_begin_irqsave(struct u6
 {
 	unsigned long flags = 0;
 
-#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
-	local_irq_save(flags);
+#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
+	if (IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_disable();
+	else
+		local_irq_save(flags);
 	write_seqcount_begin(&syncp->seq);
 #endif
 	return flags;
@@ -163,15 +170,18 @@ static inline void
 u64_stats_update_end_irqrestore(struct u64_stats_sync *syncp,
 				unsigned long flags)
 {
-#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
+#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
 	write_seqcount_end(&syncp->seq);
-	local_irq_restore(flags);
+	if (IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_enable();
+	else
+		local_irq_restore(flags);
 #endif
 }
 
 static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
 {
-#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
+#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
 	return read_seqcount_begin(&syncp->seq);
 #else
 	return 0;
@@ -180,7 +190,7 @@ static inline unsigned int __u64_stats_f
 
 static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
 {
-#if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
+#if BITS_PER_LONG == 32 && (!defined(CONFIG_SMP) && !defined(CONFIG_PREEMPT_RT))
 	preempt_disable();
 #endif
 	return __u64_stats_fetch_begin(syncp);
@@ -189,7 +199,7 @@ static inline unsigned int u64_stats_fet
 static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
 					 unsigned int start)
 {
-#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
+#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
 	return read_seqcount_retry(&syncp->seq, start);
 #else
 	return false;
@@ -199,7 +209,7 @@ static inline bool __u64_stats_fetch_ret
 static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
 					 unsigned int start)
 {
-#if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
+#if BITS_PER_LONG == 32 && (!defined(CONFIG_SMP) && !defined(CONFIG_PREEMPT_RT))
 	preempt_enable();
 #endif
 	return __u64_stats_fetch_retry(syncp, start);
@@ -213,7 +223,9 @@ static inline bool u64_stats_fetch_retry
  */
 static inline unsigned int u64_stats_fetch_begin_irq(const struct u64_stats_sync *syncp)
 {
-#if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
+#if BITS_PER_LONG == 32 && defined(CONFIG_PREEMPT_RT)
+	preempt_disable();
+#elif BITS_PER_LONG == 32 && !defined(CONFIG_SMP)
 	local_irq_disable();
 #endif
 	return __u64_stats_fetch_begin(syncp);
@@ -222,7 +234,9 @@ static inline unsigned int u64_stats_fet
 static inline bool u64_stats_fetch_retry_irq(const struct u64_stats_sync *syncp,
 					     unsigned int start)
 {
-#if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
+#if BITS_PER_LONG == 32 && defined(CONFIG_PREEMPT_RT)
+	preempt_enable();
+#elif BITS_PER_LONG == 32 && !defined(CONFIG_SMP)
 	local_irq_enable();
 #endif
 	return __u64_stats_fetch_retry(syncp, start);