u64_stats: Disable preemption on 32bit UP+SMP PREEMPT_RT during updates.

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>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/include/linux/u64_stats_sync.h b/include/linux/u64_stats_sync.h
index e8ec116c916bf7d066c99aabe14d4f79f6450da8..6ad4e9032d5384a550d75d02b7abdaa81ec3b952 100644 (file)
--- 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_stats_t *p)
 }
 #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 u64_stats_sync *syncp)
 
 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 u64_stats_sync *syncp)
 {
        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_fetch_begin(const struct u64_stats_sync *
 
 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_fetch_begin(const struct u64_stats_sync *sy
 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_retry(const struct u64_stats_sync *syncp,
 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(const struct u64_stats_sync *syncp,
  */
 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_fetch_begin_irq(const struct u64_stats_sync
 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);