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  * Protect against 64-bit values tearing on 32-bit architectures. This is
   7  * typically used for statistics read/update in different subsystems.
   8  *
   9  * Key points :
  10  *
  11  * -  Use a seqcount on 32-bit
  12  * -  The whole thing is a no-op on 64-bit architectures.
  13  *
  14  * Usage constraints:
  15  *
  16  * 1) Write side must ensure mutual exclusion, or one seqcount update could
  17  *    be lost, thus blocking readers forever.
  18  *
  19  * 2) Write side must disable preemption, or a seqcount reader can preempt the
  20  *    writer and also spin forever.
  21  *
  22  * 3) Write side must use the _irqsave() variant if other writers, or a reader,
  23  *    can be invoked from an IRQ context. On 64bit systems this variant does not
  24  *    disable interrupts.
  25  *
  26  * 4) If reader fetches several counters, there is no guarantee the whole values
  27  *    are consistent w.r.t. each other (remember point #2: seqcounts are not
  28  *    used for 64bit architectures).
  29  *
  30  * 5) Readers are allowed to sleep or be preempted/interrupted: they perform
  31  *    pure reads.
  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  *   u64_stats_add(&stats->bytes64, len); // non atomic operation
  43  *   u64_stats_inc(&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 = u64_stats_read(&stats->bytes64); // non atomic operation
  55  *         tpackets = u64_stats_read(&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
  66         seqcount_t      seq;
  67 #endif
  68 };
  69
  70 #if BITS_PER_LONG == 64
  71 #include <asm/local64.h>
  72
  73 typedef struct {
  74         local64_t       v;
  75 } u64_stats_t ;
  76
  77 static inline u64 u64_stats_read(const u64_stats_t *p)
  78 {
  79         return local64_read(&p->v);
  80 }
  81
  82 static inline void u64_stats_set(u64_stats_t *p, u64 val)
  83 {
  84         local64_set(&p->v, val);
  85 }
  86
  87 static inline void u64_stats_add(u64_stats_t *p, unsigned long val)
  88 {
  89         local64_add(val, &p->v);
  90 }
  91
  92 static inline void u64_stats_inc(u64_stats_t *p)
  93 {
  94         local64_inc(&p->v);
  95 }
  96
  97 static inline void u64_stats_init(struct u64_stats_sync *syncp) { }
  98 static inline void __u64_stats_update_begin(struct u64_stats_sync *syncp) { }
  99 static inline void __u64_stats_update_end(struct u64_stats_sync *syncp) { }
 100 static inline unsigned long __u64_stats_irqsave(void) { return 0; }
 101 static inline void __u64_stats_irqrestore(unsigned long flags) { }
 102 static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
 103 {
 104         return 0;
 105 }
 106 static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
 107                                            unsigned int start)
 108 {
 109         return false;
 110 }
 111
 112 #else /* 64 bit */
 113
 114 typedef struct {
 115         u64             v;
 116 } u64_stats_t;
 117
 118 static inline u64 u64_stats_read(const u64_stats_t *p)
 119 {
 120         return p->v;
 121 }
 122
 123 static inline void u64_stats_set(u64_stats_t *p, u64 val)
 124 {
 125         p->v = val;
 126 }
 127
 128 static inline void u64_stats_add(u64_stats_t *p, unsigned long val)
 129 {
 130         p->v += val;
 131 }
 132
 133 static inline void u64_stats_inc(u64_stats_t *p)
 134 {
 135         p->v++;
 136 }
 137
 138 #define u64_stats_init(syncp)                           \
 139         do {                                            \
 140                 struct u64_stats_sync *__s = (syncp);   \
 141                 seqcount_init(&__s->seq);               \
 142         } while (0)
 143
 144 static inline void __u64_stats_update_begin(struct u64_stats_sync *syncp)
 145 {
 146         preempt_disable_nested();
 147         write_seqcount_begin(&syncp->seq);
 148 }
 149
 150 static inline void __u64_stats_update_end(struct u64_stats_sync *syncp)
 151 {
 152         write_seqcount_end(&syncp->seq);
 153         preempt_enable_nested();
 154 }
 155
 156 static inline unsigned long __u64_stats_irqsave(void)
 157 {
 158         unsigned long flags;
 159
 160         local_irq_save(flags);
 161         return flags;
 162 }
 163
 164 static inline void __u64_stats_irqrestore(unsigned long flags)
 165 {
 166         local_irq_restore(flags);
 167 }
 168
 169 static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
 170 {
 171         return read_seqcount_begin(&syncp->seq);
 172 }
 173
 174 static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
 175                                            unsigned int start)
 176 {
 177         return read_seqcount_retry(&syncp->seq, start);
 178 }
 179 #endif /* !64 bit */
 180
 181 static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
 182 {
 183         __u64_stats_update_begin(syncp);
 184 }
 185
 186 static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
 187 {
 188         __u64_stats_update_end(syncp);
 189 }
 190
 191 static inline unsigned long u64_stats_update_begin_irqsave(struct u64_stats_sync *syncp)
 192 {
 193         unsigned long flags = __u64_stats_irqsave();
 194
 195         __u64_stats_update_begin(syncp);
 196         return flags;
 197 }
 198
 199 static inline void u64_stats_update_end_irqrestore(struct u64_stats_sync *syncp,
 200                                                    unsigned long flags)
 201 {
 202         __u64_stats_update_end(syncp);
 203         __u64_stats_irqrestore(flags);
 204 }
 205
 206 static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
 207 {
 208         return __u64_stats_fetch_begin(syncp);
 209 }
 210
 211 static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
 212                                          unsigned int start)
 213 {
 214         return __u64_stats_fetch_retry(syncp, start);
 215 }
 216
 217 #endif /* _LINUX_U64_STATS_SYNC_H */