datapath/linux/compat/include/linux/u64_stats_sync.h

   1 #ifndef _LINUX_U64_STATS_SYNC_WRAPPER_H
   2 #define _LINUX_U64_STATS_SYNC_WRAPPER_H
   3
   4 #include <linux/version.h>
   5
   6 #if defined(HAVE_U64_STATS_FETCH_BEGIN_IRQ) && \
   7     LINUX_VERSION_CODE >= KERNEL_VERSION(3,13,0)
   8 #include_next <linux/u64_stats_sync.h>
   9 #else
  10
  11 /*
  12  * To properly implement 64bits network statistics on 32bit and 64bit hosts,
  13  * we provide a synchronization point, that is a noop on 64bit or UP kernels.
  14  *
  15  * Key points :
  16  * 1) Use a seqcount on SMP 32bits, with low overhead.
  17  * 2) Whole thing is a noop on 64bit arches or UP kernels.
  18  * 3) Write side must ensure mutual exclusion or one seqcount update could
  19  *    be lost, thus blocking readers forever.
  20  *    If this synchronization point is not a mutex, but a spinlock or
  21  *    spinlock_bh() or disable_bh() :
  22  * 3.1) Write side should not sleep.
  23  * 3.2) Write side should not allow preemption.
  24  * 3.3) If applicable, interrupts should be disabled.
  25  *
  26  * 4) If reader fetches several counters, there is no guarantee the whole values
  27  *    are consistent (remember point 1) : this is a noop on 64bit arches anyway)
  28  *
  29  * 5) readers are allowed to sleep or be preempted/interrupted : They perform
  30  *    pure reads. But if they have to fetch many values, it's better to not allow
  31  *    preemptions/interruptions to avoid many retries.
  32  *
  33  * 6) If counter might be written by an interrupt, readers should block interrupts.
  34  *    (On UP, there is no seqcount_t protection, a reader allowing interrupts could
  35  *     read partial values)
  36  *
  37  * 7) For irq or softirq uses, readers can use u64_stats_fetch_begin_irq() and
  38  *    u64_stats_fetch_retry_irq() helpers
  39  *
  40  * Usage :
  41  *
  42  * Stats producer (writer) should use following template granted it already got
  43  * an exclusive access to counters (a lock is already taken, or per cpu
  44  * data is used [in a non preemptable context])
  45  *
  46  *   spin_lock_bh(...) or other synchronization to get exclusive access
  47  *   ...
  48  *   u64_stats_update_begin(&stats->syncp);
  49  *   stats->bytes64 += len; // non atomic operation
  50  *   stats->packets64++;    // non atomic operation
  51  *   u64_stats_update_end(&stats->syncp);
  52  *
  53  * While a consumer (reader) should use following template to get consistent
  54  * snapshot for each variable (but no guarantee on several ones)
  55  *
  56  * u64 tbytes, tpackets;
  57  * unsigned int start;
  58  *
  59  * do {
  60  *         start = u64_stats_fetch_begin(&stats->syncp);
  61  *         tbytes = stats->bytes64; // non atomic operation
  62  *         tpackets = stats->packets64; // non atomic operation
  63  * } while (u64_stats_fetch_retry(&stats->syncp, start));
  64  *
  65  *
  66  * Example of use in drivers/net/loopback.c, using per_cpu containers,
  67  * in BH disabled context.
  68  */
  69 #include <linux/seqlock.h>
  70
  71 struct u64_stats_sync {
  72 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
  73         seqcount_t      seq;
  74 #endif
  75 };
  76
  77 #if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
  78 # define u64_stats_init(syncp)  seqcount_init(syncp.seq)
  79 #else
  80 # define u64_stats_init(syncp)  do { } while (0)
  81 #endif
  82
  83 static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
  84 {
  85 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
  86         write_seqcount_begin(&syncp->seq);
  87 #endif
  88 }
  89
  90 static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
  91 {
  92 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
  93         write_seqcount_end(&syncp->seq);
  94 #endif
  95 }
  96
  97 static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
  98 {
  99 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 100         return read_seqcount_begin(&syncp->seq);
 101 #else
 102 #if BITS_PER_LONG==32
 103         preempt_disable();
 104 #endif
 105         return 0;
 106 #endif
 107 }
 108
 109 static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
 110                                          unsigned int start)
 111 {
 112 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 113         return read_seqcount_retry(&syncp->seq, start);
 114 #else
 115 #if BITS_PER_LONG==32
 116         preempt_enable();
 117 #endif
 118         return false;
 119 #endif
 120 }
 121
 122 /*
 123  * In case irq handlers can update u64 counters, readers can use following helpers
 124  * - SMP 32bit arches use seqcount protection, irq safe.
 125  * - UP 32bit must disable irqs.
 126  * - 64bit have no problem atomically reading u64 values, irq safe.
 127  */
 128 static inline unsigned int u64_stats_fetch_begin_irq(const struct u64_stats_sync *syncp)
 129 {
 130 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 131         return read_seqcount_begin(&syncp->seq);
 132 #else
 133 #if BITS_PER_LONG==32
 134         local_irq_disable();
 135 #endif
 136         return 0;
 137 #endif
 138 }
 139
 140 static inline bool u64_stats_fetch_retry_irq(const struct u64_stats_sync *syncp,
 141                                          unsigned int start)
 142 {
 143 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 144         return read_seqcount_retry(&syncp->seq, start);
 145 #else
 146 #if BITS_PER_LONG==32
 147         local_irq_enable();
 148 #endif
 149         return false;
 150 #endif
 151 }
 152
 153 #endif /* !HAVE_U64_STATS_FETCH_BEGIN_IRQ || kernel < 3.13 */
 154
 155 #endif /* _LINUX_U64_STATS_SYNC_WRAPPER_H */