1 #ifndef _LINUX_U64_STATS_SYNC_H 2 #define _LINUX_U64_STATS_SYNC_H 3 4 /* 5 * To properly implement 64bits network statistics on 32bit and 64bit hosts, 6 * we provide a synchronization point, that is a noop on 64bit or UP kernels. 7 * 8 * Key points : 9 * 1) Use a seqcount on SMP 32bits, with low overhead. 10 * 2) Whole thing is a noop on 64bit arches or UP kernels. 11 * 3) Write side must ensure mutual exclusion or one seqcount update could 12 * be lost, thus blocking readers forever. 13 * If this synchronization point is not a mutex, but a spinlock or 14 * spinlock_bh() or disable_bh() : 15 * 3.1) Write side should not sleep. 16 * 3.2) Write side should not allow preemption. 17 * 3.3) If applicable, interrupts should be disabled. 18 * 19 * 4) If reader fetches several counters, there is no guarantee the whole values 20 * are consistent (remember point 1) : this is a noop on 64bit arches anyway) 21 * 22 * 5) readers are allowed to sleep or be preempted/interrupted : They perform 23 * pure reads. But if they have to fetch many values, it's better to not allow 24 * preemptions/interruptions to avoid many retries. 25 * 26 * 6) If counter might be written by an interrupt, readers should block interrupts. 27 * (On UP, there is no seqcount_t protection, a reader allowing interrupts could 28 * read partial values) 29 * 30 * 7) For irq and softirq uses, readers can use u64_stats_fetch_begin_irq() and 31 * u64_stats_fetch_retry_irq() helpers 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 * stats->bytes64 += len; // non atomic operation 43 * 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 = stats->bytes64; // non atomic operation 55 * tpackets = 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 && defined(CONFIG_SMP) 66 seqcount_t seq; 67 #endif 68 }; 69 70 71 static inline void u64_stats_init(struct u64_stats_sync *syncp) 72 { 73 #if BITS_PER_LONG == 32 && defined(CONFIG_SMP) 74 seqcount_init(&syncp->seq); 75 #endif 76 } 77 78 static inline void u64_stats_update_begin(struct u64_stats_sync *syncp) 79 { 80 #if BITS_PER_LONG==32 && defined(CONFIG_SMP) 81 write_seqcount_begin(&syncp->seq); 82 #endif 83 } 84 85 static inline void u64_stats_update_end(struct u64_stats_sync *syncp) 86 { 87 #if BITS_PER_LONG==32 && defined(CONFIG_SMP) 88 write_seqcount_end(&syncp->seq); 89 #endif 90 } 91 92 static inline void u64_stats_update_begin_raw(struct u64_stats_sync *syncp) 93 { 94 #if BITS_PER_LONG==32 && defined(CONFIG_SMP) 95 raw_write_seqcount_begin(&syncp->seq); 96 #endif 97 } 98 99 static inline void u64_stats_update_end_raw(struct u64_stats_sync *syncp) 100 { 101 #if BITS_PER_LONG==32 && defined(CONFIG_SMP) 102 raw_write_seqcount_end(&syncp->seq); 103 #endif 104 } 105 106 static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp) 107 { 108 #if BITS_PER_LONG==32 && defined(CONFIG_SMP) 109 return read_seqcount_begin(&syncp->seq); 110 #else 111 #if BITS_PER_LONG==32 112 preempt_disable(); 113 #endif 114 return 0; 115 #endif 116 } 117 118 static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp, 119 unsigned int start) 120 { 121 #if BITS_PER_LONG==32 && defined(CONFIG_SMP) 122 return read_seqcount_retry(&syncp->seq, start); 123 #else 124 #if BITS_PER_LONG==32 125 preempt_enable(); 126 #endif 127 return false; 128 #endif 129 } 130 131 /* 132 * In case irq handlers can update u64 counters, readers can use following helpers 133 * - SMP 32bit arches use seqcount protection, irq safe. 134 * - UP 32bit must disable irqs. 135 * - 64bit have no problem atomically reading u64 values, irq safe. 136 */ 137 static inline unsigned int u64_stats_fetch_begin_irq(const struct u64_stats_sync *syncp) 138 { 139 #if BITS_PER_LONG==32 && defined(CONFIG_SMP) 140 return read_seqcount_begin(&syncp->seq); 141 #else 142 #if BITS_PER_LONG==32 143 local_irq_disable(); 144 #endif 145 return 0; 146 #endif 147 } 148 149 static inline bool u64_stats_fetch_retry_irq(const struct u64_stats_sync *syncp, 150 unsigned int start) 151 { 152 #if BITS_PER_LONG==32 && defined(CONFIG_SMP) 153 return read_seqcount_retry(&syncp->seq, start); 154 #else 155 #if BITS_PER_LONG==32 156 local_irq_enable(); 157 #endif 158 return false; 159 #endif 160 } 161 162 #endif /* _LINUX_U64_STATS_SYNC_H */ 163