1/* SPDX-License-Identifier: GPL-2.0 */ 2#ifndef _LINUX_U64_STATS_SYNC_H 3#define _LINUX_U64_STATS_SYNC_H 4 5/* 6 * To properly implement 64bits network statistics on 32bit and 64bit hosts, 7 * we provide a synchronization point, that is a noop on 64bit or UP kernels. 8 * 9 * Key points : 10 * 1) Use a seqcount on SMP 32bits, with low overhead. 11 * 2) Whole thing is a noop on 64bit arches or UP kernels. 12 * 3) Write side must ensure mutual exclusion or one seqcount update could 13 * be lost, thus blocking readers forever. 14 * If this synchronization point is not a mutex, but a spinlock or 15 * spinlock_bh() or disable_bh() : 16 * 3.1) Write side should not sleep. 17 * 3.2) Write side should not allow preemption. 18 * 3.3) If applicable, interrupts should be disabled. 19 * 20 * 4) If reader fetches several counters, there is no guarantee the whole values 21 * are consistent (remember point 1) : this is a noop on 64bit arches anyway) 22 * 23 * 5) readers are allowed to sleep or be preempted/interrupted : They perform 24 * pure reads. But if they have to fetch many values, it's better to not allow 25 * preemptions/interruptions to avoid many retries. 26 * 27 * 6) If counter might be written by an interrupt, readers should block interrupts. 28 * (On UP, there is no seqcount_t protection, a reader allowing interrupts could 29 * read partial values) 30 * 31 * 7) For irq and softirq uses, readers can use u64_stats_fetch_begin_irq() and 32 * u64_stats_fetch_retry_irq() helpers 33 * 34 * Usage : 35 * 36 * Stats producer (writer) should use following template granted it already got 37 * an exclusive access to counters (a lock is already taken, or per cpu 38 * data is used [in a non preemptable context]) 39 * 40 * spin_lock_bh(...) or other synchronization to get exclusive access 41 * ... 42 * u64_stats_update_begin(&stats->syncp); 43 * stats->bytes64 += len; // non atomic operation 44 * stats->packets64++; // non atomic operation 45 * u64_stats_update_end(&stats->syncp); 46 * 47 * While a consumer (reader) should use following template to get consistent 48 * snapshot for each variable (but no guarantee on several ones) 49 * 50 * u64 tbytes, tpackets; 51 * unsigned int start; 52 * 53 * do { 54 * start = u64_stats_fetch_begin(&stats->syncp); 55 * tbytes = stats->bytes64; // non atomic operation 56 * tpackets = stats->packets64; // non atomic operation 57 * } while (u64_stats_fetch_retry(&stats->syncp, start)); 58 * 59 * 60 * Example of use in drivers/net/loopback.c, using per_cpu containers, 61 * in BH disabled context. 62 */ 63#include <linux/seqlock.h> 64 65struct u64_stats_sync { 66#if BITS_PER_LONG==32 && defined(CONFIG_SMP) 67 seqcount_t seq; 68#endif 69}; 70 71 72static inline void u64_stats_init(struct u64_stats_sync *syncp) 73{ 74#if BITS_PER_LONG == 32 && defined(CONFIG_SMP) 75 seqcount_init(&syncp->seq); 76#endif 77} 78 79static inline void u64_stats_update_begin(struct u64_stats_sync *syncp) 80{ 81#if BITS_PER_LONG==32 && defined(CONFIG_SMP) 82 write_seqcount_begin(&syncp->seq); 83#endif 84} 85 86static inline void u64_stats_update_end(struct u64_stats_sync *syncp) 87{ 88#if BITS_PER_LONG==32 && defined(CONFIG_SMP) 89 write_seqcount_end(&syncp->seq); 90#endif 91} 92 93static inline unsigned long 94u64_stats_update_begin_irqsave(struct u64_stats_sync *syncp) 95{ 96 unsigned long flags = 0; 97 98#if BITS_PER_LONG==32 && defined(CONFIG_SMP) 99 local_irq_save(flags); 100 write_seqcount_begin(&syncp->seq); 101#endif 102 return flags; 103} 104 105static inline void 106u64_stats_update_end_irqrestore(struct u64_stats_sync *syncp, 107 unsigned long flags) 108{ 109#if BITS_PER_LONG==32 && defined(CONFIG_SMP) 110 write_seqcount_end(&syncp->seq); 111 local_irq_restore(flags); 112#endif 113} 114 115static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp) 116{ 117#if BITS_PER_LONG==32 && defined(CONFIG_SMP) 118 return read_seqcount_begin(&syncp->seq); 119#else 120 return 0; 121#endif 122} 123 124static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp) 125{ 126#if BITS_PER_LONG==32 && !defined(CONFIG_SMP) 127 preempt_disable(); 128#endif 129 return __u64_stats_fetch_begin(syncp); 130} 131 132static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp, 133 unsigned int start) 134{ 135#if BITS_PER_LONG==32 && defined(CONFIG_SMP) 136 return read_seqcount_retry(&syncp->seq, start); 137#else 138 return false; 139#endif 140} 141 142static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp, 143 unsigned int start) 144{ 145#if BITS_PER_LONG==32 && !defined(CONFIG_SMP) 146 preempt_enable(); 147#endif 148 return __u64_stats_fetch_retry(syncp, start); 149} 150 151/* 152 * In case irq handlers can update u64 counters, readers can use following helpers 153 * - SMP 32bit arches use seqcount protection, irq safe. 154 * - UP 32bit must disable irqs. 155 * - 64bit have no problem atomically reading u64 values, irq safe. 156 */ 157static inline unsigned int u64_stats_fetch_begin_irq(const struct u64_stats_sync *syncp) 158{ 159#if BITS_PER_LONG==32 && !defined(CONFIG_SMP) 160 local_irq_disable(); 161#endif 162 return __u64_stats_fetch_begin(syncp); 163} 164 165static inline bool u64_stats_fetch_retry_irq(const struct u64_stats_sync *syncp, 166 unsigned int start) 167{ 168#if BITS_PER_LONG==32 && !defined(CONFIG_SMP) 169 local_irq_enable(); 170#endif 171 return __u64_stats_fetch_retry(syncp, start); 172} 173 174#endif /* _LINUX_U64_STATS_SYNC_H */ 175