1 /* SPDX-License-Identifier: GPL-2.0 */ 2 #ifndef _LINUX_U64_STATS_SYNC_H 3 #define _LINUX_U64_STATS_SYNC_H 4 5 /* 6 * Protect against 64-bit values tearing on 32-bit architectures. This is 7 * typically used for statistics read/update in different subsystems. 8 * 9 * Key points : 10 * 11 * - Use a seqcount on 32-bit 12 * - The whole thing is a no-op on 64-bit architectures. 13 * 14 * Usage constraints: 15 * 16 * 1) Write side must ensure mutual exclusion, or one seqcount update could 17 * be lost, thus blocking readers forever. 18 * 19 * 2) Write side must disable preemption, or a seqcount reader can preempt the 20 * writer and also spin forever. 21 * 22 * 3) Write side must use the _irqsave() variant if other writers, or a reader, 23 * can be invoked from an IRQ context. On 64bit systems this variant does not 24 * disable interrupts. 25 * 26 * 4) If reader fetches several counters, there is no guarantee the whole values 27 * are consistent w.r.t. each other (remember point #2: seqcounts are not 28 * used for 64bit architectures). 29 * 30 * 5) Readers are allowed to sleep or be preempted/interrupted: they perform 31 * pure reads. 32 * 33 * Usage : 34 * 35 * Stats producer (writer) should use following template granted it already got 36 * an exclusive access to counters (a lock is already taken, or per cpu 37 * data is used [in a non preemptable context]) 38 * 39 * spin_lock_bh(...) or other synchronization to get exclusive access 40 * ... 41 * u64_stats_update_begin(&stats->syncp); 42 * u64_stats_add(&stats->bytes64, len); // non atomic operation 43 * u64_stats_inc(&stats->packets64); // non atomic operation 44 * u64_stats_update_end(&stats->syncp); 45 * 46 * While a consumer (reader) should use following template to get consistent 47 * snapshot for each variable (but no guarantee on several ones) 48 * 49 * u64 tbytes, tpackets; 50 * unsigned int start; 51 * 52 * do { 53 * start = u64_stats_fetch_begin(&stats->syncp); 54 * tbytes = u64_stats_read(&stats->bytes64); // non atomic operation 55 * tpackets = u64_stats_read(&stats->packets64); // non atomic operation 56 * } while (u64_stats_fetch_retry(&stats->syncp, start)); 57 * 58 * 59 * Example of use in drivers/net/loopback.c, using per_cpu containers, 60 * in BH disabled context. 61 */ 62 #include <linux/seqlock.h> 63 64 struct u64_stats_sync { 65 #if BITS_PER_LONG == 32 66 seqcount_t seq; 67 #endif 68 }; 69 70 #if BITS_PER_LONG == 64 71 #include <asm/local64.h> 72 73 typedef struct { 74 local64_t v; 75 } u64_stats_t ; 76 77 static inline u64 u64_stats_read(const u64_stats_t *p) 78 { 79 return local64_read(&p->v); 80 } 81 82 static inline void u64_stats_set(u64_stats_t *p, u64 val) 83 { 84 local64_set(&p->v, val); 85 } 86 87 static inline void u64_stats_add(u64_stats_t *p, unsigned long val) 88 { 89 local64_add(val, &p->v); 90 } 91 92 static inline void u64_stats_inc(u64_stats_t *p) 93 { 94 local64_inc(&p->v); 95 } 96 97 static inline void u64_stats_init(struct u64_stats_sync *syncp) { } 98 static inline void __u64_stats_update_begin(struct u64_stats_sync *syncp) { } 99 static inline void __u64_stats_update_end(struct u64_stats_sync *syncp) { } 100 static inline unsigned long __u64_stats_irqsave(void) { return 0; } 101 static inline void __u64_stats_irqrestore(unsigned long flags) { } 102 static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp) 103 { 104 return 0; 105 } 106 static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp, 107 unsigned int start) 108 { 109 return false; 110 } 111 112 #else /* 64 bit */ 113 114 typedef struct { 115 u64 v; 116 } u64_stats_t; 117 118 static inline u64 u64_stats_read(const u64_stats_t *p) 119 { 120 return p->v; 121 } 122 123 static inline void u64_stats_set(u64_stats_t *p, u64 val) 124 { 125 p->v = val; 126 } 127 128 static inline void u64_stats_add(u64_stats_t *p, unsigned long val) 129 { 130 p->v += val; 131 } 132 133 static inline void u64_stats_inc(u64_stats_t *p) 134 { 135 p->v++; 136 } 137 138 static inline void u64_stats_init(struct u64_stats_sync *syncp) 139 { 140 seqcount_init(&syncp->seq); 141 } 142 143 static inline void __u64_stats_update_begin(struct u64_stats_sync *syncp) 144 { 145 preempt_disable_nested(); 146 write_seqcount_begin(&syncp->seq); 147 } 148 149 static inline void __u64_stats_update_end(struct u64_stats_sync *syncp) 150 { 151 write_seqcount_end(&syncp->seq); 152 preempt_enable_nested(); 153 } 154 155 static inline unsigned long __u64_stats_irqsave(void) 156 { 157 unsigned long flags; 158 159 local_irq_save(flags); 160 return flags; 161 } 162 163 static inline void __u64_stats_irqrestore(unsigned long flags) 164 { 165 local_irq_restore(flags); 166 } 167 168 static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp) 169 { 170 return read_seqcount_begin(&syncp->seq); 171 } 172 173 static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp, 174 unsigned int start) 175 { 176 return read_seqcount_retry(&syncp->seq, start); 177 } 178 #endif /* !64 bit */ 179 180 static inline void u64_stats_update_begin(struct u64_stats_sync *syncp) 181 { 182 __u64_stats_update_begin(syncp); 183 } 184 185 static inline void u64_stats_update_end(struct u64_stats_sync *syncp) 186 { 187 __u64_stats_update_end(syncp); 188 } 189 190 static inline unsigned long u64_stats_update_begin_irqsave(struct u64_stats_sync *syncp) 191 { 192 unsigned long flags = __u64_stats_irqsave(); 193 194 __u64_stats_update_begin(syncp); 195 return flags; 196 } 197 198 static inline void u64_stats_update_end_irqrestore(struct u64_stats_sync *syncp, 199 unsigned long flags) 200 { 201 __u64_stats_update_end(syncp); 202 __u64_stats_irqrestore(flags); 203 } 204 205 static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp) 206 { 207 return __u64_stats_fetch_begin(syncp); 208 } 209 210 static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp, 211 unsigned int start) 212 { 213 return __u64_stats_fetch_retry(syncp, start); 214 } 215 216 #endif /* _LINUX_U64_STATS_SYNC_H */ 217