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    Subject[PATCH 8/9] u64_stats: Streamline the implementation
    Date
    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
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      Last update: 2022-08-17 18:29    [W:2.079 / U:0.532 seconds]
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