[PATCH 8/9] u64_stats: Streamline the implementation

Sebastian Andrzej Siewior
2022-08-17T16:28:20Z

From: Thomas Gleixner <tglx@linutronix.de>

The u64 stats code handles 3 different cases:

- 32bit UP
- 32bit SMP
- 64bit

with an unreadable #ifdef maze, which was recently expanded with PREEMPT_RT
conditionals.

Reduce it to two cases (32bit and 64bit) and drop the optimization for
32bit UP as suggested by Linus.

Use the new preempt_disable/enable_nested() helpers to get rid of the
CONFIG_PREEMPT_RT conditionals.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: netdev@vger.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
include/linux/u64_stats_sync.h | 147 +++++++++++++++------------------
1 file changed, 65 insertions(+), 82 deletions(-)

diff --git a/include/linux/u64_stats_sync.h b/include/linux/u64_stats_sync.h
index 6ad4e9032d538..46040d66334a8 100644
--- a/include/linux/u64_stats_sync.h
+++ b/include/linux/u64_stats_sync.h
@@ -8,7 +8,7 @@
*
* Key points :
*
- * - Use a seqcount on 32-bit SMP, only disable preemption for 32-bit UP.
+ * - Use a seqcount on 32-bit
* - The whole thing is a no-op on 64-bit architectures.
*
* Usage constraints:
@@ -20,7 +20,8 @@
* writer and also spin forever.
*
* 3) Write side must use the _irqsave() variant if other writers, or a reader,
- * can be invoked from an IRQ context.
+ * can be invoked from an IRQ context. On 64bit systems this variant does not
+ * disable interrupts.
*
* 4) If reader fetches several counters, there is no guarantee the whole values
* are consistent w.r.t. each other (remember point #2: seqcounts are not
@@ -29,11 +30,6 @@
* 5) Readers are allowed to sleep or be preempted/interrupted: they perform
* pure reads.
*
- * 6) Readers must use both u64_stats_fetch_{begin,retry}_irq() if the stats
- * might be updated from a hardirq or softirq context (remember point #1:
- * seqcounts are not used for UP kernels). 32-bit UP stat readers could read
- * corrupted 64-bit values otherwise.
- *
* Usage :
*
* Stats producer (writer) should use following template granted it already got
@@ -66,7 +62,7 @@
#include <linux/seqlock.h>

struct u64_stats_sync {
-#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
+#if BITS_PER_LONG == 32
seqcount_t seq;
#endif
};
@@ -98,7 +94,22 @@ static inline void u64_stats_inc(u64_stats_t *p)
local64_inc(&p->v);
}

-#else
+static inline void u64_stats_init(struct u64_stats_sync *syncp) { }
+static inline void __u64_stats_update_begin(struct u64_stats_sync *syncp) { }
+static inline void __u64_stats_update_end(struct u64_stats_sync *syncp) { }
+static inline unsigned long __u64_stats_irqsave(void) { return 0; }
+static inline void __u64_stats_irqrestore(unsigned long flags) { }
+static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
+{
+ return 0;
+}
+static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
+ unsigned int start)
+{
+ return false;
+}
+
+#else /* 64 bit */

typedef struct {
u64 v;
@@ -123,123 +134,95 @@ static inline void u64_stats_inc(u64_stats_t *p)
{
p->v++;
}
-#endif

-#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)
{
+ seqcount_init(&syncp->seq);
}
-#endif

-static inline void u64_stats_update_begin(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) || defined(CONFIG_PREEMPT_RT))
- if (IS_ENABLED(CONFIG_PREEMPT_RT))
- preempt_disable();
+ preempt_disable_nested();
write_seqcount_begin(&syncp->seq);
-#endif
}

-static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
+static inline void __u64_stats_update_end(struct u64_stats_sync *syncp)
{
-#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
+ preempt_enable_nested();
}

-static inline unsigned long
-u64_stats_update_begin_irqsave(struct u64_stats_sync *syncp)
+static inline unsigned long __u64_stats_irqsave(void)
{
- unsigned long flags = 0;
+ unsigned long 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
+ local_irq_save(flags);
return flags;
}

-static inline void
-u64_stats_update_end_irqrestore(struct u64_stats_sync *syncp,
- unsigned long flags)
+static inline void __u64_stats_irqrestore(unsigned long flags)
{
-#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();
- else
- local_irq_restore(flags);
-#endif
+ local_irq_restore(flags);
}

static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
{
-#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
return read_seqcount_begin(&syncp->seq);
-#else
- return 0;
-#endif
+}
+
+static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
+ unsigned int start)
+{
+ return read_seqcount_retry(&syncp->seq, start);
+}
+#endif /* !64 bit */
+
+static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
+{
+ __u64_stats_update_begin(syncp);
+}
+
+static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
+{
+ __u64_stats_update_end(syncp);
+}
+
+static inline unsigned long u64_stats_update_begin_irqsave(struct u64_stats_sync *syncp)
+{
+ unsigned long flags = __u64_stats_irqsave();
+
+ __u64_stats_update_begin(syncp);
+ return flags;
+}
+
+static inline void u64_stats_update_end_irqrestore(struct u64_stats_sync *syncp,
+ unsigned long flags)
+{
+ __u64_stats_update_end(syncp);
+ __u64_stats_irqrestore(flags);
}

static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
{
-#if BITS_PER_LONG == 32 && (!defined(CONFIG_SMP) && !defined(CONFIG_PREEMPT_RT))
- preempt_disable();
-#endif
return __u64_stats_fetch_begin(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) || defined(CONFIG_PREEMPT_RT))
- return read_seqcount_retry(&syncp->seq, start);
-#else
- return false;
-#endif
-}
-
static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
unsigned int start)
{
-#if BITS_PER_LONG == 32 && (!defined(CONFIG_SMP) && !defined(CONFIG_PREEMPT_RT))
- preempt_enable();
-#endif
return __u64_stats_fetch_retry(syncp, start);
}

-/*
- * In case irq handlers can update u64 counters, readers can use following helpers
- * - SMP 32bit arches use seqcount protection, irq safe.
- * - UP 32bit must disable irqs.
- * - 64bit have no problem atomically reading u64 values, irq safe.
- */
+/* Obsolete interfaces */
static inline unsigned int u64_stats_fetch_begin_irq(const struct u64_stats_sync *syncp)
{
-#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);
+ return u64_stats_fetch_begin(syncp);
}

static inline bool u64_stats_fetch_retry_irq(const struct u64_stats_sync *syncp,
unsigned int start)
{
-#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);
+ return u64_stats_fetch_retry(syncp, start);
}

#endif /* _LINUX_U64_STATS_SYNC_H */
--
2.37.2