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Linux/include/linux/u64_stats_sync.h

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  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  *   stats->bytes64 += len; // non atomic operation
 44  *   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 = stats->bytes64; // non atomic operation
 56  *         tpackets = 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 
 72 static inline void u64_stats_init(struct u64_stats_sync *syncp)
 73 {
 74 #if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
 75         seqcount_init(&syncp->seq);
 76 #endif
 77 }
 78 
 79 static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
 80 {
 81 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 82         write_seqcount_begin(&syncp->seq);
 83 #endif
 84 }
 85 
 86 static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
 87 {
 88 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 89         write_seqcount_end(&syncp->seq);
 90 #endif
 91 }
 92 
 93 static inline unsigned long
 94 u64_stats_update_begin_irqsave(struct u64_stats_sync *syncp)
 95 {
 96         unsigned long flags = 0;
 97 
 98 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 99         local_irq_save(flags);
100         write_seqcount_begin(&syncp->seq);
101 #endif
102         return flags;
103 }
104 
105 static inline void
106 u64_stats_update_end_irqrestore(struct u64_stats_sync *syncp,
107                                 unsigned long flags)
108 {
109 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
110         write_seqcount_end(&syncp->seq);
111         local_irq_restore(flags);
112 #endif
113 }
114 
115 static inline unsigned int __u64_stats_fetch_begin(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         return 0;
121 #endif
122 }
123 
124 static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
125 {
126 #if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
127         preempt_disable();
128 #endif
129         return __u64_stats_fetch_begin(syncp);
130 }
131 
132 static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
133                                          unsigned int start)
134 {
135 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
136         return read_seqcount_retry(&syncp->seq, start);
137 #else
138         return false;
139 #endif
140 }
141 
142 static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
143                                          unsigned int start)
144 {
145 #if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
146         preempt_enable();
147 #endif
148         return __u64_stats_fetch_retry(syncp, start);
149 }
150 
151 /*
152  * In case irq handlers can update u64 counters, readers can use following helpers
153  * - SMP 32bit arches use seqcount protection, irq safe.
154  * - UP 32bit must disable irqs.
155  * - 64bit have no problem atomically reading u64 values, irq safe.
156  */
157 static inline unsigned int u64_stats_fetch_begin_irq(const struct u64_stats_sync *syncp)
158 {
159 #if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
160         local_irq_disable();
161 #endif
162         return __u64_stats_fetch_begin(syncp);
163 }
164 
165 static inline bool u64_stats_fetch_retry_irq(const struct u64_stats_sync *syncp,
166                                              unsigned int start)
167 {
168 #if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
169         local_irq_enable();
170 #endif
171         return __u64_stats_fetch_retry(syncp, start);
172 }
173 
174 #endif /* _LINUX_U64_STATS_SYNC_H */
175 

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