1 /* SPDX-License-Identifier: GPL-2.0 */
2 #ifndef _LINUX_U64_STATS_SYNC_H
3 #define _LINUX_U64_STATS_SYNC_H
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.
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.
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)
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.
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)
31 * 7) For irq and softirq uses, readers can use u64_stats_fetch_begin_irq() and
32 * u64_stats_fetch_retry_irq() helpers
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])
40 * spin_lock_bh(...) or other synchronization to get exclusive access
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);
47 * While a consumer (reader) should use following template to get consistent
48 * snapshot for each variable (but no guarantee on several ones)
50 * u64 tbytes, tpackets;
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));
60 * Example of use in drivers/net/loopback.c, using per_cpu containers,
61 * in BH disabled context.
63 #include <linux/seqlock.h>
65 struct u64_stats_sync {
66 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
72 static inline void u64_stats_init(struct u64_stats_sync *syncp)
74 #if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
75 seqcount_init(&syncp->seq);
79 static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
81 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
82 write_seqcount_begin(&syncp->seq);
86 static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
88 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
89 write_seqcount_end(&syncp->seq);
93 static inline unsigned long
94 u64_stats_update_begin_irqsave(struct u64_stats_sync *syncp)
96 unsigned long flags = 0;
98 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
99 local_irq_save(flags);
100 write_seqcount_begin(&syncp->seq);
106 u64_stats_update_end_irqrestore(struct u64_stats_sync *syncp,
109 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
110 write_seqcount_end(&syncp->seq);
111 local_irq_restore(flags);
115 static inline void u64_stats_update_begin_raw(struct u64_stats_sync *syncp)
117 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
118 raw_write_seqcount_begin(&syncp->seq);
122 static inline void u64_stats_update_end_raw(struct u64_stats_sync *syncp)
124 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
125 raw_write_seqcount_end(&syncp->seq);
129 static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
131 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
132 return read_seqcount_begin(&syncp->seq);
138 static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
140 #if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
143 return __u64_stats_fetch_begin(syncp);
146 static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
149 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
150 return read_seqcount_retry(&syncp->seq, start);
156 static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
159 #if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
162 return __u64_stats_fetch_retry(syncp, start);
166 * In case irq handlers can update u64 counters, readers can use following helpers
167 * - SMP 32bit arches use seqcount protection, irq safe.
168 * - UP 32bit must disable irqs.
169 * - 64bit have no problem atomically reading u64 values, irq safe.
171 static inline unsigned int u64_stats_fetch_begin_irq(const struct u64_stats_sync *syncp)
173 #if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
176 return __u64_stats_fetch_begin(syncp);
179 static inline bool u64_stats_fetch_retry_irq(const struct u64_stats_sync *syncp,
182 #if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
185 return __u64_stats_fetch_retry(syncp, start);
188 #endif /* _LINUX_U64_STATS_SYNC_H */