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 softirq uses, readers can use u64_stats_fetch_begin_bh() and 31 * u64_stats_fetch_retry_bh() 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 static inline void u64_stats_update_begin(struct u64_stats_sync *syncp) 71 { 72 #if BITS_PER_LONG==32 && defined(CONFIG_SMP) 73 write_seqcount_begin(&syncp->seq); 74 #endif 75 } 76 77 static inline void u64_stats_update_end(struct u64_stats_sync *syncp) 78 { 79 #if BITS_PER_LONG==32 && defined(CONFIG_SMP) 80 write_seqcount_end(&syncp->seq); 81 #endif 82 } 83 84 static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp) 85 { 86 #if BITS_PER_LONG==32 && defined(CONFIG_SMP) 87 return read_seqcount_begin(&syncp->seq); 88 #else 89 #if BITS_PER_LONG==32 90 preempt_disable(); 91 #endif 92 return 0; 93 #endif 94 } 95 96 static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp, 97 unsigned int start) 98 { 99 #if BITS_PER_LONG==32 && defined(CONFIG_SMP) 100 return read_seqcount_retry(&syncp->seq, start); 101 #else 102 #if BITS_PER_LONG==32 103 preempt_enable(); 104 #endif 105 return false; 106 #endif 107 } 108 109 /* 110 * In case softirq handlers can update u64 counters, readers can use following helpers 111 * - SMP 32bit arches use seqcount protection, irq safe. 112 * - UP 32bit must disable BH. 113 * - 64bit have no problem atomically reading u64 values, irq safe. 114 */ 115 static inline unsigned int u64_stats_fetch_begin_bh(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 #if BITS_PER_LONG==32 121 local_bh_disable(); 122 #endif 123 return 0; 124 #endif 125 } 126 127 static inline bool u64_stats_fetch_retry_bh(const struct u64_stats_sync *syncp, 128 unsigned int start) 129 { 130 #if BITS_PER_LONG==32 && defined(CONFIG_SMP) 131 return read_seqcount_retry(&syncp->seq, start); 132 #else 133 #if BITS_PER_LONG==32 134 local_bh_enable(); 135 #endif 136 return false; 137 #endif 138 } 139 140 #endif /* _LINUX_U64_STATS_SYNC_H */ 141