xref: /linux/include/linux/u64_stats_sync.h (revision 3a07362fab1653d3aca31a9155c8cc776138fd02)
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 
64 struct 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 
73 typedef struct {
74 	local64_t	v;
75 } u64_stats_t ;
76 
u64_stats_read(const u64_stats_t * p)77 static inline u64 u64_stats_read(const u64_stats_t *p)
78 {
79 	return local64_read(&p->v);
80 }
81 
u64_stats_set(u64_stats_t * p,u64 val)82 static inline void u64_stats_set(u64_stats_t *p, u64 val)
83 {
84 	local64_set(&p->v, val);
85 }
86 
u64_stats_add(u64_stats_t * p,unsigned long val)87 static inline void u64_stats_add(u64_stats_t *p, unsigned long val)
88 {
89 	local64_add(val, &p->v);
90 }
91 
u64_stats_inc(u64_stats_t * p)92 static inline void u64_stats_inc(u64_stats_t *p)
93 {
94 	local64_inc(&p->v);
95 }
96 
u64_stats_init(struct u64_stats_sync * syncp)97 static inline void u64_stats_init(struct u64_stats_sync *syncp) { }
__u64_stats_update_begin(struct u64_stats_sync * syncp)98 static inline void __u64_stats_update_begin(struct u64_stats_sync *syncp) { }
__u64_stats_update_end(struct u64_stats_sync * syncp)99 static inline void __u64_stats_update_end(struct u64_stats_sync *syncp) { }
__u64_stats_irqsave(void)100 static inline unsigned long __u64_stats_irqsave(void) { return 0; }
__u64_stats_irqrestore(unsigned long flags)101 static inline void __u64_stats_irqrestore(unsigned long flags) { }
__u64_stats_fetch_begin(const struct u64_stats_sync * syncp)102 static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
103 {
104 	return 0;
105 }
__u64_stats_fetch_retry(const struct u64_stats_sync * syncp,unsigned int start)106 static 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 
114 typedef struct {
115 	u64		v;
116 } u64_stats_t;
117 
u64_stats_read(const u64_stats_t * p)118 static inline u64 u64_stats_read(const u64_stats_t *p)
119 {
120 	return p->v;
121 }
122 
u64_stats_set(u64_stats_t * p,u64 val)123 static inline void u64_stats_set(u64_stats_t *p, u64 val)
124 {
125 	p->v = val;
126 }
127 
u64_stats_add(u64_stats_t * p,unsigned long val)128 static inline void u64_stats_add(u64_stats_t *p, unsigned long val)
129 {
130 	p->v += val;
131 }
132 
u64_stats_inc(u64_stats_t * p)133 static 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 
__u64_stats_update_begin(struct u64_stats_sync * syncp)144 static inline void __u64_stats_update_begin(struct u64_stats_sync *syncp)
145 {
146 	preempt_disable_nested();
147 	write_seqcount_begin(&syncp->seq);
148 }
149 
__u64_stats_update_end(struct u64_stats_sync * syncp)150 static inline void __u64_stats_update_end(struct u64_stats_sync *syncp)
151 {
152 	write_seqcount_end(&syncp->seq);
153 	preempt_enable_nested();
154 }
155 
__u64_stats_irqsave(void)156 static inline unsigned long __u64_stats_irqsave(void)
157 {
158 	unsigned long flags;
159 
160 	local_irq_save(flags);
161 	return flags;
162 }
163 
__u64_stats_irqrestore(unsigned long flags)164 static inline void __u64_stats_irqrestore(unsigned long flags)
165 {
166 	local_irq_restore(flags);
167 }
168 
__u64_stats_fetch_begin(const struct u64_stats_sync * syncp)169 static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
170 {
171 	return read_seqcount_begin(&syncp->seq);
172 }
173 
__u64_stats_fetch_retry(const struct u64_stats_sync * syncp,unsigned int start)174 static 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 
u64_stats_update_begin(struct u64_stats_sync * syncp)181 static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
182 {
183 	__u64_stats_update_begin(syncp);
184 }
185 
u64_stats_update_end(struct u64_stats_sync * syncp)186 static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
187 {
188 	__u64_stats_update_end(syncp);
189 }
190 
u64_stats_update_begin_irqsave(struct u64_stats_sync * syncp)191 static 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 
u64_stats_update_end_irqrestore(struct u64_stats_sync * syncp,unsigned long flags)199 static 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 
u64_stats_fetch_begin(const struct u64_stats_sync * syncp)206 static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
207 {
208 	return __u64_stats_fetch_begin(syncp);
209 }
210 
u64_stats_fetch_retry(const struct u64_stats_sync * syncp,unsigned int start)211 static 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