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 * Usage : 27 * 28 * Stats producer (writer) should use following template granted it already got 29 * an exclusive access to counters (a lock is already taken, or per cpu 30 * data is used [in a non preemptable context]) 31 * 32 * spin_lock_bh(...) or other synchronization to get exclusive access 33 * ... 34 * u64_stats_update_begin(&stats->syncp); 35 * stats->bytes64 += len; // non atomic operation 36 * stats->packets64++; // non atomic operation 37 * u64_stats_update_end(&stats->syncp); 38 * 39 * While a consumer (reader) should use following template to get consistent 40 * snapshot for each variable (but no guarantee on several ones) 41 * 42 * u64 tbytes, tpackets; 43 * unsigned int start; 44 * 45 * do { 46 * start = u64_stats_fetch_begin(&stats->syncp); 47 * tbytes = stats->bytes64; // non atomic operation 48 * tpackets = stats->packets64; // non atomic operation 49 * } while (u64_stats_fetch_retry(&stats->lock, syncp)); 50 * 51 * 52 * Example of use in drivers/net/loopback.c, using per_cpu containers, 53 * in BH disabled context. 54 */ 55 #include <linux/seqlock.h> 56 57 #if BITS_PER_LONG==32 && defined(CONFIG_SMP) 58 struct u64_stats_sync { 59 seqcount_t seq; 60 }; 61 62 static void inline u64_stats_update_begin(struct u64_stats_sync *syncp) 63 { 64 write_seqcount_begin(&syncp->seq); 65 } 66 67 static void inline u64_stats_update_end(struct u64_stats_sync *syncp) 68 { 69 write_seqcount_end(&syncp->seq); 70 } 71 72 static unsigned int inline u64_stats_fetch_begin(const struct u64_stats_sync *syncp) 73 { 74 return read_seqcount_begin(&syncp->seq); 75 } 76 77 static bool inline u64_stats_fetch_retry(const struct u64_stats_sync *syncp, 78 unsigned int start) 79 { 80 return read_seqcount_retry(&syncp->seq, start); 81 } 82 83 #else 84 struct u64_stats_sync { 85 }; 86 87 static void inline u64_stats_update_begin(struct u64_stats_sync *syncp) 88 { 89 } 90 91 static void inline u64_stats_update_end(struct u64_stats_sync *syncp) 92 { 93 } 94 95 static unsigned int inline u64_stats_fetch_begin(const struct u64_stats_sync *syncp) 96 { 97 return 0; 98 } 99 100 static bool inline u64_stats_fetch_retry(const struct u64_stats_sync *syncp, 101 unsigned int start) 102 { 103 return false; 104 } 105 #endif 106 107 #endif /* _LINUX_U64_STATS_SYNC_H */ 108