1/* SPDX-License-Identifier: GPL-2.0 */
2#ifndef _LINUX_U64_STATS_SYNC_H
3#define _LINUX_U64_STATS_SYNC_H
4
5/*
6 * Protect against 64-bit values tearing on 32-bit architectures. This is
7 * typically used for statistics read/update in different subsystems.
8 *
9 * Key points :
10 *
11 * -  Use a seqcount on 32-bit
12 * -  The whole thing is a no-op on 64-bit architectures.
13 *
14 * Usage constraints:
15 *
16 * 1) Write side must ensure mutual exclusion, or one seqcount update could
17 *    be lost, thus blocking readers forever.
18 *
19 * 2) Write side must disable preemption, or a seqcount reader can preempt the
20 *    writer and also spin forever.
21 *
22 * 3) Write side must use the _irqsave() variant if other writers, or a reader,
23 *    can be invoked from an IRQ context. On 64bit systems this variant does not
24 *    disable interrupts.
25 *
26 * 4) If reader fetches several counters, there is no guarantee the whole values
27 *    are consistent w.r.t. each other (remember point #2: seqcounts are not
28 *    used for 64bit architectures).
29 *
30 * 5) Readers are allowed to sleep or be preempted/interrupted: they perform
31 *    pure reads.
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 *   u64_stats_add(&stats->bytes64, len); // non atomic operation
43 *   u64_stats_inc(&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 = u64_stats_read(&stats->bytes64); // non atomic operation
55 *         tpackets = u64_stats_read(&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
64struct u64_stats_sync {
65#if BITS_PER_LONG == 32
66	seqcount_t	seq;
67#endif
68};
69
70#if BITS_PER_LONG == 64
71#include <asm/local64.h>
72
73typedef struct {
74	local64_t	v;
75} u64_stats_t ;
76
77static inline u64 u64_stats_read(const u64_stats_t *p)
78{
79	return local64_read(&p->v);
80}
81
82static inline void u64_stats_set(u64_stats_t *p, u64 val)
83{
84	local64_set(&p->v, val);
85}
86
87static inline void u64_stats_add(u64_stats_t *p, unsigned long val)
88{
89	local64_add(val, &p->v);
90}
91
92static inline void u64_stats_inc(u64_stats_t *p)
93{
94	local64_inc(&p->v);
95}
96
97static inline void u64_stats_init(struct u64_stats_sync *syncp) { }
98static inline void __u64_stats_update_begin(struct u64_stats_sync *syncp) { }
99static inline void __u64_stats_update_end(struct u64_stats_sync *syncp) { }
100static inline unsigned long __u64_stats_irqsave(void) { return 0; }
101static inline void __u64_stats_irqrestore(unsigned long flags) { }
102static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
103{
104	return 0;
105}
106static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
107					   unsigned int start)
108{
109	return false;
110}
111
112#else /* 64 bit */
113
114typedef struct {
115	u64		v;
116} u64_stats_t;
117
118static inline u64 u64_stats_read(const u64_stats_t *p)
119{
120	return p->v;
121}
122
123static inline void u64_stats_set(u64_stats_t *p, u64 val)
124{
125	p->v = val;
126}
127
128static inline void u64_stats_add(u64_stats_t *p, unsigned long val)
129{
130	p->v += val;
131}
132
133static inline void u64_stats_inc(u64_stats_t *p)
134{
135	p->v++;
136}
137
138#define u64_stats_init(syncp)				\
139	do {						\
140		struct u64_stats_sync *__s = (syncp);	\
141		seqcount_init(&__s->seq);		\
142	} while (0)
143
144static inline void __u64_stats_update_begin(struct u64_stats_sync *syncp)
145{
146	preempt_disable_nested();
147	write_seqcount_begin(&syncp->seq);
148}
149
150static inline void __u64_stats_update_end(struct u64_stats_sync *syncp)
151{
152	write_seqcount_end(&syncp->seq);
153	preempt_enable_nested();
154}
155
156static inline unsigned long __u64_stats_irqsave(void)
157{
158	unsigned long flags;
159
160	local_irq_save(flags);
161	return flags;
162}
163
164static inline void __u64_stats_irqrestore(unsigned long flags)
165{
166	local_irq_restore(flags);
167}
168
169static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
170{
171	return read_seqcount_begin(&syncp->seq);
172}
173
174static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
175					   unsigned int start)
176{
177	return read_seqcount_retry(&syncp->seq, start);
178}
179#endif /* !64 bit */
180
181static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
182{
183	__u64_stats_update_begin(syncp);
184}
185
186static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
187{
188	__u64_stats_update_end(syncp);
189}
190
191static inline unsigned long u64_stats_update_begin_irqsave(struct u64_stats_sync *syncp)
192{
193	unsigned long flags = __u64_stats_irqsave();
194
195	__u64_stats_update_begin(syncp);
196	return flags;
197}
198
199static inline void u64_stats_update_end_irqrestore(struct u64_stats_sync *syncp,
200						   unsigned long flags)
201{
202	__u64_stats_update_end(syncp);
203	__u64_stats_irqrestore(flags);
204}
205
206static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
207{
208	return __u64_stats_fetch_begin(syncp);
209}
210
211static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
212					 unsigned int start)
213{
214	return __u64_stats_fetch_retry(syncp, start);
215}
216
217#endif /* _LINUX_U64_STATS_SYNC_H */
218