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 * Usage : 31 * 32 * Stats producer (writer) should use following template granted it already got 33 * an exclusive access to counters (a lock is already taken, or per cpu 34 * data is used [in a non preemptable context]) 35 * 36 * spin_lock_bh(...) or other synchronization to get exclusive access 37 * ... 38 * u64_stats_update_begin(&stats->syncp); 39 * stats->bytes64 += len; // non atomic operation 40 * stats->packets64++; // non atomic operation 41 * u64_stats_update_end(&stats->syncp); 42 * 43 * While a consumer (reader) should use following template to get consistent 44 * snapshot for each variable (but no guarantee on several ones) 45 * 46 * u64 tbytes, tpackets; 47 * unsigned int start; 48 * 49 * do { 50 * start = u64_stats_fetch_begin(&stats->syncp); 51 * tbytes = stats->bytes64; // non atomic operation 52 * tpackets = stats->packets64; // non atomic operation 53 * } while (u64_stats_fetch_retry(&stats->syncp, start)); 54 * 55 * 56 * Example of use in drivers/net/loopback.c, using per_cpu containers, 57 * in BH disabled context. 58 */ 59 #include <linux/seqlock.h> 60 61 #if BITS_PER_LONG==32 && defined(CONFIG_SMP) 62 struct u64_stats_sync { 63 seqcount_t seq; 64 }; 65 66 static void inline u64_stats_update_begin(struct u64_stats_sync *syncp) 67 { 68 write_seqcount_begin(&syncp->seq); 69 } 70 71 static void inline u64_stats_update_end(struct u64_stats_sync *syncp) 72 { 73 write_seqcount_end(&syncp->seq); 74 } 75 76 static unsigned int inline u64_stats_fetch_begin(const struct u64_stats_sync *syncp) 77 { 78 return read_seqcount_begin(&syncp->seq); 79 } 80 81 static bool inline u64_stats_fetch_retry(const struct u64_stats_sync *syncp, 82 unsigned int start) 83 { 84 return read_seqcount_retry(&syncp->seq, start); 85 } 86 87 #else 88 struct u64_stats_sync { 89 }; 90 91 static void inline u64_stats_update_begin(struct u64_stats_sync *syncp) 92 { 93 } 94 95 static void inline u64_stats_update_end(struct u64_stats_sync *syncp) 96 { 97 } 98 99 static unsigned int inline u64_stats_fetch_begin(const struct u64_stats_sync *syncp) 100 { 101 return 0; 102 } 103 104 static bool inline u64_stats_fetch_retry(const struct u64_stats_sync *syncp, 105 unsigned int start) 106 { 107 return false; 108 } 109 #endif 110 111 #endif /* _LINUX_U64_STATS_SYNC_H */ 112