include/linux/u64_stats_sync.h

   1 /* SPDX-License-Identifier: GPL-2.0 */
   2 #ifndef _LINUX_U64_STATS_SYNC_H
   3 #define _LINUX_U64_STATS_SYNC_H
   4
   5 /*
   6  * To properly implement 64bits network statistics on 32bit and 64bit hosts,
   7  * we provide a synchronization point, that is a noop on 64bit or UP kernels.
   8  *
   9  * Key points :
  10  * 1) Use a seqcount on SMP 32bits, with low overhead.
  11  * 2) Whole thing is a noop on 64bit arches or UP kernels.
  12  * 3) Write side must ensure mutual exclusion or one seqcount update could
  13  *    be lost, thus blocking readers forever.
  14  *    If this synchronization point is not a mutex, but a spinlock or
  15  *    spinlock_bh() or disable_bh() :
  16  * 3.1) Write side should not sleep.
  17  * 3.2) Write side should not allow preemption.
  18  * 3.3) If applicable, interrupts should be disabled.
  19  *
  20  * 4) If reader fetches several counters, there is no guarantee the whole values
  21  *    are consistent (remember point 1) : this is a noop on 64bit arches anyway)
  22  *
  23  * 5) readers are allowed to sleep or be preempted/interrupted : They perform
  24  *    pure reads. But if they have to fetch many values, it's better to not allow
  25  *    preemptions/interruptions to avoid many retries.
  26  *
  27  * 6) If counter might be written by an interrupt, readers should block interrupts.
  28  *    (On UP, there is no seqcount_t protection, a reader allowing interrupts could
  29  *     read partial values)
  30  *
  31  * 7) For irq and softirq uses, readers can use u64_stats_fetch_begin_irq() and
  32  *    u64_stats_fetch_retry_irq() helpers
  33  *
  34  * Usage :
  35  *
  36  * Stats producer (writer) should use following template granted it already got
  37  * an exclusive access to counters (a lock is already taken, or per cpu
  38  * data is used [in a non preemptable context])
  39  *
  40  *   spin_lock_bh(...) or other synchronization to get exclusive access
  41  *   ...
  42  *   u64_stats_update_begin(&stats->syncp);
  43  *   u64_stats_add(&stats->bytes64, len); // non atomic operation
  44  *   u64_stats_inc(&stats->packets64);    // non atomic operation
  45  *   u64_stats_update_end(&stats->syncp);
  46  *
  47  * While a consumer (reader) should use following template to get consistent
  48  * snapshot for each variable (but no guarantee on several ones)
  49  *
  50  * u64 tbytes, tpackets;
  51  * unsigned int start;
  52  *
  53  * do {
  54  *         start = u64_stats_fetch_begin(&stats->syncp);
  55  *         tbytes = u64_stats_read(&stats->bytes64); // non atomic operation
  56  *         tpackets = u64_stats_read(&stats->packets64); // non atomic operation
  57  * } while (u64_stats_fetch_retry(&stats->syncp, start));
  58  *
  59  *
  60  * Example of use in drivers/net/loopback.c, using per_cpu containers,
  61  * in BH disabled context.
  62  */
  63 #include <linux/seqlock.h>
  64
  65 struct u64_stats_sync {
  66 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
  67         seqcount_t      seq;
  68 #endif
  69 };
  70
  71 #if BITS_PER_LONG == 64
  72 #include <asm/local64.h>
  73
  74 typedef struct {
  75         local64_t       v;
  76 } u64_stats_t ;
  77
  78 static inline u64 u64_stats_read(const u64_stats_t *p)
  79 {
  80         return local64_read(&p->v);
  81 }
  82
  83 static inline void u64_stats_add(u64_stats_t *p, unsigned long val)
  84 {
  85         local64_add(val, &p->v);
  86 }
  87
  88 static inline void u64_stats_inc(u64_stats_t *p)
  89 {
  90         local64_inc(&p->v);
  91 }
  92
  93 #else
  94
  95 typedef struct {
  96         u64             v;
  97 } u64_stats_t;
  98
  99 static inline u64 u64_stats_read(const u64_stats_t *p)
 100 {
 101         return p->v;
 102 }
 103
 104 static inline void u64_stats_add(u64_stats_t *p, unsigned long val)
 105 {
 106         p->v += val;
 107 }
 108
 109 static inline void u64_stats_inc(u64_stats_t *p)
 110 {
 111         p->v++;
 112 }
 113 #endif
 114
 115 static inline void u64_stats_init(struct u64_stats_sync *syncp)
 116 {
 117 #if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
 118         seqcount_init(&syncp->seq);
 119 #endif
 120 }
 121
 122 static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
 123 {
 124 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 125         write_seqcount_begin(&syncp->seq);
 126 #endif
 127 }
 128
 129 static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
 130 {
 131 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 132         write_seqcount_end(&syncp->seq);
 133 #endif
 134 }
 135
 136 static inline unsigned long
 137 u64_stats_update_begin_irqsave(struct u64_stats_sync *syncp)
 138 {
 139         unsigned long flags = 0;
 140
 141 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 142         local_irq_save(flags);
 143         write_seqcount_begin(&syncp->seq);
 144 #endif
 145         return flags;
 146 }
 147
 148 static inline void
 149 u64_stats_update_end_irqrestore(struct u64_stats_sync *syncp,
 150                                 unsigned long flags)
 151 {
 152 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 153         write_seqcount_end(&syncp->seq);
 154         local_irq_restore(flags);
 155 #endif
 156 }
 157
 158 static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
 159 {
 160 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 161         return read_seqcount_begin(&syncp->seq);
 162 #else
 163         return 0;
 164 #endif
 165 }
 166
 167 static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
 168 {
 169 #if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
 170         preempt_disable();
 171 #endif
 172         return __u64_stats_fetch_begin(syncp);
 173 }
 174
 175 static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
 176                                          unsigned int start)
 177 {
 178 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 179         return read_seqcount_retry(&syncp->seq, start);
 180 #else
 181         return false;
 182 #endif
 183 }
 184
 185 static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
 186                                          unsigned int start)
 187 {
 188 #if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
 189         preempt_enable();
 190 #endif
 191         return __u64_stats_fetch_retry(syncp, start);
 192 }
 193
 194 /*
 195  * In case irq handlers can update u64 counters, readers can use following helpers
 196  * - SMP 32bit arches use seqcount protection, irq safe.
 197  * - UP 32bit must disable irqs.
 198  * - 64bit have no problem atomically reading u64 values, irq safe.
 199  */
 200 static inline unsigned int u64_stats_fetch_begin_irq(const struct u64_stats_sync *syncp)
 201 {
 202 #if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
 203         local_irq_disable();
 204 #endif
 205         return __u64_stats_fetch_begin(syncp);
 206 }
 207
 208 static inline bool u64_stats_fetch_retry_irq(const struct u64_stats_sync *syncp,
 209                                              unsigned int start)
 210 {
 211 #if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
 212         local_irq_enable();
 213 #endif
 214         return __u64_stats_fetch_retry(syncp, start);
 215 }
 216
 217 #endif /* _LINUX_U64_STATS_SYNC_H */