name : u64_stats_sync.h /* SPDX-License-Identifier: GPL-2.0 */ #ifndef _LINUX_U64_STATS_SYNC_H #define _LINUX_U64_STATS_SYNC_H /* * Protect against 64-bit values tearing on 32-bit architectures. This is * typically used for statistics read/update in different subsystems. * * Key points : * * - Use a seqcount on 32-bit * - The whole thing is a no-op on 64-bit architectures. * * Usage constraints: * * 1) Write side must ensure mutual exclusion, or one seqcount update could * be lost, thus blocking readers forever. * * 2) Write side must disable preemption, or a seqcount reader can preempt the * 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. 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 * used for 64bit architectures). * * 5) Readers are allowed to sleep or be preempted/interrupted: they perform * pure reads. * * 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); * u64_stats_add(&stats->bytes64, len); // non atomic operation * u64_stats_inc(&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 = u64_stats_read(&stats->bytes64); // non atomic operation * tpackets = u64_stats_read(&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 seqcount_t seq; #endif }; #if BITS_PER_LONG == 64 #include <asm/local64.h> typedef struct { local64_t v; } u64_stats_t ; static inline u64 u64_stats_read(const u64_stats_t *p) { return local64_read(&p->v); } static inline void u64_stats_set(u64_stats_t *p, u64 val) { local64_set(&p->v, val); } static inline void u64_stats_add(u64_stats_t *p, unsigned long val) { local64_add(val, &p->v); } static inline void u64_stats_inc(u64_stats_t *p) { local64_inc(&p->v); } 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; } u64_stats_t; static inline u64 u64_stats_read(const u64_stats_t *p) { return p->v; } static inline void u64_stats_set(u64_stats_t *p, u64 val) { p->v = val; } static inline void u64_stats_add(u64_stats_t *p, unsigned long val) { p->v += val; } static inline void u64_stats_inc(u64_stats_t *p) { p->v++; } static inline void u64_stats_init(struct u64_stats_sync *syncp) { seqcount_init(&syncp->seq); } static inline void __u64_stats_update_begin(struct u64_stats_sync *syncp) { preempt_disable_nested(); write_seqcount_begin(&syncp->seq); } static inline void __u64_stats_update_end(struct u64_stats_sync *syncp) { write_seqcount_end(&syncp->seq); preempt_enable_nested(); } static inline unsigned long __u64_stats_irqsave(void) { unsigned long flags; local_irq_save(flags); return flags; } static inline void __u64_stats_irqrestore(unsigned long flags) { local_irq_restore(flags); } static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp) { return read_seqcount_begin(&syncp->seq); } 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) { return __u64_stats_fetch_begin(syncp); } static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp, unsigned int start) { return __u64_stats_fetch_retry(syncp, start); } #endif /* _LINUX_U64_STATS_SYNC_H */