summaryrefslogtreecommitdiff
path: root/include/linux/u64_stats_sync.h
blob: 9de5c10293f593fac6f64458d78f413c3c9fe26c (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
/* SPDX-License-Identifier: GPL-2.0 */
#ifndef _LINUX_U64_STATS_SYNC_H
#define _LINUX_U64_STATS_SYNC_H

/*
 * To properly implement 64bits network statistics on 32bit and 64bit hosts,
 * we provide a synchronization point, that is a noop on 64bit or UP kernels.
 *
 * Key points :
 * 1) Use a seqcount on SMP 32bits, with low overhead.
 * 2) Whole thing is a noop on 64bit arches or UP kernels.
 * 3) Write side must ensure mutual exclusion or one seqcount update could
 *    be lost, thus blocking readers forever.
 *    If this synchronization point is not a mutex, but a spinlock or
 *    spinlock_bh() or disable_bh() :
 * 3.1) Write side should not sleep.
 * 3.2) Write side should not allow preemption.
 * 3.3) If applicable, interrupts should be disabled.
 *
 * 4) If reader fetches several counters, there is no guarantee the whole values
 *    are consistent (remember point 1) : this is a noop on 64bit arches anyway)
 *
 * 5) readers are allowed to sleep or be preempted/interrupted : They perform
 *    pure reads. But if they have to fetch many values, it's better to not allow
 *    preemptions/interruptions to avoid many retries.
 *
 * 6) If counter might be written by an interrupt, readers should block interrupts.
 *    (On UP, there is no seqcount_t protection, a reader allowing interrupts could
 *     read partial values)
 *
 * 7) For irq and softirq uses, readers can use u64_stats_fetch_begin_irq() and
 *    u64_stats_fetch_retry_irq() helpers
 *
 * Usage :
 *
 * Stats producer (writer) should use following template granted it already got
 * an exclusive access to counters (a lock is already taken, or per cpu
 * data is used [in a non preemptable context])
 *
 *   spin_lock_bh(...) or other synchronization to get exclusive access
 *   ...
 *   u64_stats_update_begin(&stats->syncp);
 *   u64_stats_add(&stats->bytes64, len); // non atomic operation
 *   u64_stats_inc(&stats->packets64);    // non atomic operation
 *   u64_stats_update_end(&stats->syncp);
 *
 * While a consumer (reader) should use following template to get consistent
 * snapshot for each variable (but no guarantee on several ones)
 *
 * u64 tbytes, tpackets;
 * unsigned int start;
 *
 * do {
 *         start = u64_stats_fetch_begin(&stats->syncp);
 *         tbytes = u64_stats_read(&stats->bytes64); // non atomic operation
 *         tpackets = u64_stats_read(&stats->packets64); // non atomic operation
 * } while (u64_stats_fetch_retry(&stats->syncp, start));
 *
 *
 * Example of use in drivers/net/loopback.c, using per_cpu containers,
 * in BH disabled context.
 */
#include <linux/seqlock.h>

struct u64_stats_sync {
#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
	seqcount_t	seq;
#endif
};

#if BITS_PER_LONG == 64
#include <asm/local64.h>

typedef struct {
	local64_t	v;
} u64_stats_t ;

static inline u64 u64_stats_read(const u64_stats_t *p)
{
	return local64_read(&p->v);
}

static inline void u64_stats_add(u64_stats_t *p, unsigned long val)
{
	local64_add(val, &p->v);
}

static inline void u64_stats_inc(u64_stats_t *p)
{
	local64_inc(&p->v);
}

#else

typedef struct {
	u64		v;
} u64_stats_t;

static inline u64 u64_stats_read(const u64_stats_t *p)
{
	return p->v;
}

static inline void u64_stats_add(u64_stats_t *p, unsigned long val)
{
	p->v += val;
}

static inline void u64_stats_inc(u64_stats_t *p)
{
	p->v++;
}
#endif

static inline void u64_stats_init(struct u64_stats_sync *syncp)
{
#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
	seqcount_init(&syncp->seq);
#endif
}

static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
{
#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
	write_seqcount_begin(&syncp->seq);
#endif
}

static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
{
#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
	write_seqcount_end(&syncp->seq);
#endif
}

static inline unsigned long
u64_stats_update_begin_irqsave(struct u64_stats_sync *syncp)
{
	unsigned long flags = 0;

#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
	local_irq_save(flags);
	write_seqcount_begin(&syncp->seq);
#endif
	return flags;
}

static inline void
u64_stats_update_end_irqrestore(struct u64_stats_sync *syncp,
				unsigned long flags)
{
#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
	write_seqcount_end(&syncp->seq);
	local_irq_restore(flags);
#endif
}

static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
{
#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
	return read_seqcount_begin(&syncp->seq);
#else
	return 0;
#endif
}

static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
{
#if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
	preempt_disable();
#endif
	return __u64_stats_fetch_begin(syncp);
}

static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
					 unsigned int start)
{
#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
	return read_seqcount_retry(&syncp->seq, start);
#else
	return false;
#endif
}

static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
					 unsigned int start)
{
#if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
	preempt_enable();
#endif
	return __u64_stats_fetch_retry(syncp, start);
}

/*
 * In case irq handlers can update u64 counters, readers can use following helpers
 * - SMP 32bit arches use seqcount protection, irq safe.
 * - UP 32bit must disable irqs.
 * - 64bit have no problem atomically reading u64 values, irq safe.
 */
static inline unsigned int u64_stats_fetch_begin_irq(const struct u64_stats_sync *syncp)
{
#if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
	local_irq_disable();
#endif
	return __u64_stats_fetch_begin(syncp);
}

static inline bool u64_stats_fetch_retry_irq(const struct u64_stats_sync *syncp,
					     unsigned int start)
{
#if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
	local_irq_enable();
#endif
	return __u64_stats_fetch_retry(syncp, start);
}

#endif /* _LINUX_U64_STATS_SYNC_H */