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* [PATCH net-next] u64_stats: Disable preemption on 32bit UP+SMP PREEMPT_RT during updates.
@ 2021-12-10 20:29 Sebastian Andrzej Siewior
  2021-12-13 12:50 ` patchwork-bot+netdevbpf
  0 siblings, 1 reply; 2+ messages in thread
From: Sebastian Andrzej Siewior @ 2021-12-10 20:29 UTC (permalink / raw)
  To: linux-kernel, netdev
  Cc: Peter Zijlstra, Thomas Gleixner, Jakub Kicinski, David S. Miller

On PREEMPT_RT the seqcount_t for synchronisation is required on 32bit
architectures even on UP because the softirq (and the threaded IRQ handler) can
be preempted.

With the seqcount_t for synchronisation, a reader with higher priority can
preempt the writer and then spin endlessly in read_seqcount_begin() while the
writer can't make progress.

To avoid such a lock up on PREEMPT_RT the writer must disable preemption during
the update. There is no need to disable interrupts because no writer is using
this API in hard-IRQ context on PREEMPT_RT.

Disable preemption on 32bit-RT within the u64_stats write section.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@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);

^ permalink raw reply	[flat|nested] 2+ messages in thread
* Re: [PATCH net-next] u64_stats: Disable preemption on 32bit UP+SMP PREEMPT_RT during updates.
  2021-12-10 20:29 [PATCH net-next] u64_stats: Disable preemption on 32bit UP+SMP PREEMPT_RT during updates Sebastian Andrzej Siewior
@ 2021-12-13 12:50 ` patchwork-bot+netdevbpf
  0 siblings, 0 replies; 2+ messages in thread
From: patchwork-bot+netdevbpf @ 2021-12-13 12:50 UTC (permalink / raw)
  To: Sebastian Andrzej Siewior; +Cc: linux-kernel, netdev, peterz, tglx, kuba, davem

Hello:

This patch was applied to netdev/net-next.git (master)
by David S. Miller <davem@davemloft.net>:

On Fri, 10 Dec 2021 21:29:59 +0100 you wrote:
> On PREEMPT_RT the seqcount_t for synchronisation is required on 32bit
> architectures even on UP because the softirq (and the threaded IRQ handler) can
> be preempted.
> 
> With the seqcount_t for synchronisation, a reader with higher priority can
> preempt the writer and then spin endlessly in read_seqcount_begin() while the
> writer can't make progress.
> 
> [...]

Here is the summary with links:
  - [net-next] u64_stats: Disable preemption on 32bit UP+SMP PREEMPT_RT during updates.
    https://git.kernel.org/netdev/net-next/c/3c118547f87e

You are awesome, thank you!
-- 
Deet-doot-dot, I am a bot.
https://korg.docs.kernel.org/patchwork/pwbot.html



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2021-12-10 20:29 [PATCH net-next] u64_stats: Disable preemption on 32bit UP+SMP PREEMPT_RT during updates Sebastian Andrzej Siewior
2021-12-13 12:50 ` patchwork-bot+netdevbpf

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