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Linux/include/linux/u64_stats_sync.h

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  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  * 7) For softirq uses, readers can use u64_stats_fetch_begin_bh() and
 31  *    u64_stats_fetch_retry_bh() helpers
 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  *   stats->bytes64 += len; // non atomic operation
 43  *   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 = stats->bytes64; // non atomic operation
 55  *         tpackets = 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 && defined(CONFIG_SMP)
 66         seqcount_t      seq;
 67 #endif
 68 };
 69 
 70 static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
 71 {
 72 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 73         write_seqcount_begin(&syncp->seq);
 74 #endif
 75 }
 76 
 77 static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
 78 {
 79 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 80         write_seqcount_end(&syncp->seq);
 81 #endif
 82 }
 83 
 84 static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
 85 {
 86 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 87         return read_seqcount_begin(&syncp->seq);
 88 #else
 89 #if BITS_PER_LONG==32
 90         preempt_disable();
 91 #endif
 92         return 0;
 93 #endif
 94 }
 95 
 96 static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
 97                                          unsigned int start)
 98 {
 99 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
100         return read_seqcount_retry(&syncp->seq, start);
101 #else
102 #if BITS_PER_LONG==32
103         preempt_enable();
104 #endif
105         return false;
106 #endif
107 }
108 
109 /*
110  * In case softirq handlers can update u64 counters, readers can use following helpers
111  * - SMP 32bit arches use seqcount protection, irq safe.
112  * - UP 32bit must disable BH.
113  * - 64bit have no problem atomically reading u64 values, irq safe.
114  */
115 static inline unsigned int u64_stats_fetch_begin_bh(const struct u64_stats_sync *syncp)
116 {
117 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
118         return read_seqcount_begin(&syncp->seq);
119 #else
120 #if BITS_PER_LONG==32
121         local_bh_disable();
122 #endif
123         return 0;
124 #endif
125 }
126 
127 static inline bool u64_stats_fetch_retry_bh(const struct u64_stats_sync *syncp,
128                                          unsigned int start)
129 {
130 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
131         return read_seqcount_retry(&syncp->seq, start);
132 #else
133 #if BITS_PER_LONG==32
134         local_bh_enable();
135 #endif
136         return false;
137 #endif
138 }
139 
140 #endif /* _LINUX_U64_STATS_SYNC_H */
141 

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