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 65 struct u64_stats_sync { 66 #if BITS_PER_LONG==32 && defined(CONFIG_SMP) 67 seqcount_t seq; 68 #endif 69 }; 70 71 72 static 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 79 static 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 86 static 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 93 static inline void u64_stats_update_begin_raw(struct u64_stats_sync *syncp) 94 { 95 #if BITS_PER_LONG==32 && defined(CONFIG_SMP) 96 raw_write_seqcount_begin(&syncp->seq); 97 #endif 98 } 99 100 static inline void u64_stats_update_end_raw(struct u64_stats_sync *syncp) 101 { 102 #if BITS_PER_LONG==32 && defined(CONFIG_SMP) 103 raw_write_seqcount_end(&syncp->seq); 104 #endif 105 } 106 107 static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp) 108 { 109 #if BITS_PER_LONG==32 && defined(CONFIG_SMP) 110 return read_seqcount_begin(&syncp->seq); 111 #else 112 return 0; 113 #endif 114 } 115 116 static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp) 117 { 118 #if BITS_PER_LONG==32 && !defined(CONFIG_SMP) 119 preempt_disable(); 120 #endif 121 return __u64_stats_fetch_begin(syncp); 122 } 123 124 static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp, 125 unsigned int start) 126 { 127 #if BITS_PER_LONG==32 && defined(CONFIG_SMP) 128 return read_seqcount_retry(&syncp->seq, start); 129 #else 130 return false; 131 #endif 132 } 133 134 static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp, 135 unsigned int start) 136 { 137 #if BITS_PER_LONG==32 && !defined(CONFIG_SMP) 138 preempt_enable(); 139 #endif 140 return __u64_stats_fetch_retry(syncp, start); 141 } 142 143 /* 144 * In case irq handlers can update u64 counters, readers can use following helpers 145 * - SMP 32bit arches use seqcount protection, irq safe. 146 * - UP 32bit must disable irqs. 147 * - 64bit have no problem atomically reading u64 values, irq safe. 148 */ 149 static inline unsigned int u64_stats_fetch_begin_irq(const struct u64_stats_sync *syncp) 150 { 151 #if BITS_PER_LONG==32 && !defined(CONFIG_SMP) 152 local_irq_disable(); 153 #endif 154 return __u64_stats_fetch_begin(syncp); 155 } 156 157 static inline bool u64_stats_fetch_retry_irq(const struct u64_stats_sync *syncp, 158 unsigned int start) 159 { 160 #if BITS_PER_LONG==32 && !defined(CONFIG_SMP) 161 local_irq_enable(); 162 #endif 163 return __u64_stats_fetch_retry(syncp, start); 164 } 165 166 #endif /* _LINUX_U64_STATS_SYNC_H */ 167