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u64_stats: Streamline the implementation
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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>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220825164131.402717-9-bigeasy@linutronix.de
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Thomas Gleixner committed Sep 19, 2022
1 parent 9458e0a commit 44b0c29
Showing 1 changed file with 64 additions and 81 deletions.
145 changes: 64 additions & 81 deletions include/linux/u64_stats_sync.h
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Expand Up @@ -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:
Expand All @@ -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
Expand All @@ -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
Expand Down Expand Up @@ -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
};
Expand Down Expand Up @@ -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;
Expand All @@ -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 unsigned int u64_stats_fetch_begin(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) && !defined(CONFIG_PREEMPT_RT))
preempt_disable();
#endif
return __u64_stats_fetch_begin(syncp);
return read_seqcount_retry(&syncp->seq, start);
}
#endif /* !64 bit */

static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
unsigned int start)
static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
{
#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
return read_seqcount_retry(&syncp->seq, start);
#else
return false;
#endif
__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)
{
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))
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 */

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