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Diffstat (limited to 'kernel/include/linux/u64_stats_sync.h')
-rw-r--r-- | kernel/include/linux/u64_stats_sync.h | 147 |
1 files changed, 147 insertions, 0 deletions
diff --git a/kernel/include/linux/u64_stats_sync.h b/kernel/include/linux/u64_stats_sync.h new file mode 100644 index 000000000..4b4439e75 --- /dev/null +++ b/kernel/include/linux/u64_stats_sync.h @@ -0,0 +1,147 @@ +#ifndef _LINUX_U64_STATS_SYNC_H +#define _LINUX_U64_STATS_SYNC_H + +/* + * To properly implement 64bits network statistics on 32bit and 64bit hosts, + * we provide a synchronization point, that is a noop on 64bit or UP kernels. + * + * Key points : + * 1) Use a seqcount on SMP 32bits, with low overhead. + * 2) Whole thing is a noop on 64bit arches or UP kernels. + * 3) Write side must ensure mutual exclusion or one seqcount update could + * be lost, thus blocking readers forever. + * If this synchronization point is not a mutex, but a spinlock or + * spinlock_bh() or disable_bh() : + * 3.1) Write side should not sleep. + * 3.2) Write side should not allow preemption. + * 3.3) If applicable, interrupts should be disabled. + * + * 4) If reader fetches several counters, there is no guarantee the whole values + * are consistent (remember point 1) : this is a noop on 64bit arches anyway) + * + * 5) readers are allowed to sleep or be preempted/interrupted : They perform + * pure reads. But if they have to fetch many values, it's better to not allow + * preemptions/interruptions to avoid many retries. + * + * 6) If counter might be written by an interrupt, readers should block interrupts. + * (On UP, there is no seqcount_t protection, a reader allowing interrupts could + * read partial values) + * + * 7) For irq and softirq uses, readers can use u64_stats_fetch_begin_irq() and + * u64_stats_fetch_retry_irq() helpers + * + * Usage : + * + * Stats producer (writer) should use following template granted it already got + * an exclusive access to counters (a lock is already taken, or per cpu + * data is used [in a non preemptable context]) + * + * spin_lock_bh(...) or other synchronization to get exclusive access + * ... + * u64_stats_update_begin(&stats->syncp); + * stats->bytes64 += len; // non atomic operation + * stats->packets64++; // non atomic operation + * u64_stats_update_end(&stats->syncp); + * + * While a consumer (reader) should use following template to get consistent + * snapshot for each variable (but no guarantee on several ones) + * + * u64 tbytes, tpackets; + * unsigned int start; + * + * do { + * start = u64_stats_fetch_begin(&stats->syncp); + * tbytes = stats->bytes64; // non atomic operation + * tpackets = stats->packets64; // non atomic operation + * } while (u64_stats_fetch_retry(&stats->syncp, start)); + * + * + * Example of use in drivers/net/loopback.c, using per_cpu containers, + * in BH disabled context. + */ +#include <linux/seqlock.h> + +struct u64_stats_sync { +#if BITS_PER_LONG==32 && defined(CONFIG_SMP) + seqcount_t seq; +#endif +}; + + +#if BITS_PER_LONG == 32 && defined(CONFIG_SMP) +# define u64_stats_init(syncp) seqcount_init(syncp.seq) +#else +# define u64_stats_init(syncp) do { } while (0) +#endif + +static inline void u64_stats_update_begin(struct u64_stats_sync *syncp) +{ +#if BITS_PER_LONG==32 && defined(CONFIG_SMP) + 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) + write_seqcount_end(&syncp->seq); +#endif +} + +static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp) +{ +#if BITS_PER_LONG==32 && defined(CONFIG_SMP) + return read_seqcount_begin(&syncp->seq); +#else +#if BITS_PER_LONG==32 + preempt_disable(); +#endif + return 0; +#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) + return read_seqcount_retry(&syncp->seq, start); +#else +#if BITS_PER_LONG==32 + preempt_enable(); +#endif + return false; +#endif +} + +/* + * 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. + */ +static inline unsigned int u64_stats_fetch_begin_irq(const struct u64_stats_sync *syncp) +{ +#if BITS_PER_LONG==32 && defined(CONFIG_SMP) + return read_seqcount_begin(&syncp->seq); +#else +#if BITS_PER_LONG==32 + local_irq_disable(); +#endif + return 0; +#endif +} + +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) + return read_seqcount_retry(&syncp->seq, start); +#else +#if BITS_PER_LONG==32 + local_irq_enable(); +#endif + return false; +#endif +} + +#endif /* _LINUX_U64_STATS_SYNC_H */ |