1 ====================================== 1 ======================================
2 Sequence counters and sequential locks 2 Sequence counters and sequential locks
3 ====================================== 3 ======================================
4 4
5 Introduction 5 Introduction
6 ============ 6 ============
7 7
8 Sequence counters are a reader-writer consiste 8 Sequence counters are a reader-writer consistency mechanism with
9 lockless readers (read-only retry loops), and 9 lockless readers (read-only retry loops), and no writer starvation. They
10 are used for data that's rarely written to (e. 10 are used for data that's rarely written to (e.g. system time), where the
11 reader wants a consistent set of information a 11 reader wants a consistent set of information and is willing to retry if
12 that information changes. 12 that information changes.
13 13
14 A data set is consistent when the sequence cou 14 A data set is consistent when the sequence count at the beginning of the
15 read side critical section is even and the sam 15 read side critical section is even and the same sequence count value is
16 read again at the end of the critical section. 16 read again at the end of the critical section. The data in the set must
17 be copied out inside the read side critical se 17 be copied out inside the read side critical section. If the sequence
18 count has changed between the start and the en 18 count has changed between the start and the end of the critical section,
19 the reader must retry. 19 the reader must retry.
20 20
21 Writers increment the sequence count at the st 21 Writers increment the sequence count at the start and the end of their
22 critical section. After starting the critical 22 critical section. After starting the critical section the sequence count
23 is odd and indicates to the readers that an up 23 is odd and indicates to the readers that an update is in progress. At
24 the end of the write side critical section the 24 the end of the write side critical section the sequence count becomes
25 even again which lets readers make progress. 25 even again which lets readers make progress.
26 26
27 A sequence counter write side critical section 27 A sequence counter write side critical section must never be preempted
28 or interrupted by read side sections. Otherwis 28 or interrupted by read side sections. Otherwise the reader will spin for
29 the entire scheduler tick due to the odd seque 29 the entire scheduler tick due to the odd sequence count value and the
30 interrupted writer. If that reader belongs to 30 interrupted writer. If that reader belongs to a real-time scheduling
31 class, it can spin forever and the kernel will 31 class, it can spin forever and the kernel will livelock.
32 32
33 This mechanism cannot be used if the protected 33 This mechanism cannot be used if the protected data contains pointers,
34 as the writer can invalidate a pointer that th 34 as the writer can invalidate a pointer that the reader is following.
35 35
36 36
37 .. _seqcount_t: 37 .. _seqcount_t:
38 38
39 Sequence counters (``seqcount_t``) 39 Sequence counters (``seqcount_t``)
40 ================================== 40 ==================================
41 41
42 This is the raw counting mechanism, which does !! 42 This is the the raw counting mechanism, which does not protect against
43 multiple writers. Write side critical section 43 multiple writers. Write side critical sections must thus be serialized
44 by an external lock. 44 by an external lock.
45 45
46 If the write serialization primitive is not im 46 If the write serialization primitive is not implicitly disabling
47 preemption, preemption must be explicitly disa 47 preemption, preemption must be explicitly disabled before entering the
48 write side section. If the read section can be 48 write side section. If the read section can be invoked from hardirq or
49 softirq contexts, interrupts or bottom halves 49 softirq contexts, interrupts or bottom halves must also be respectively
50 disabled before entering the write section. 50 disabled before entering the write section.
51 51
52 If it's desired to automatically handle the se 52 If it's desired to automatically handle the sequence counter
53 requirements of writer serialization and non-p 53 requirements of writer serialization and non-preemptibility, use
54 :ref:`seqlock_t` instead. 54 :ref:`seqlock_t` instead.
55 55
56 Initialization:: 56 Initialization::
57 57
58 /* dynamic */ 58 /* dynamic */
59 seqcount_t foo_seqcount; 59 seqcount_t foo_seqcount;
60 seqcount_init(&foo_seqcount); 60 seqcount_init(&foo_seqcount);
61 61
62 /* static */ 62 /* static */
63 static seqcount_t foo_seqcount = SEQCN 63 static seqcount_t foo_seqcount = SEQCNT_ZERO(foo_seqcount);
64 64
65 /* C99 struct init */ 65 /* C99 struct init */
66 struct { 66 struct {
67 .seq = SEQCNT_ZERO(foo.seq), 67 .seq = SEQCNT_ZERO(foo.seq),
68 } foo; 68 } foo;
69 69
70 Write path:: 70 Write path::
71 71
72 /* Serialized context with disabled pr 72 /* Serialized context with disabled preemption */
73 73
74 write_seqcount_begin(&foo_seqcount); 74 write_seqcount_begin(&foo_seqcount);
75 75
76 /* ... [[write-side critical section]] 76 /* ... [[write-side critical section]] ... */
77 77
78 write_seqcount_end(&foo_seqcount); 78 write_seqcount_end(&foo_seqcount);
79 79
80 Read path:: 80 Read path::
81 81
82 do { 82 do {
83 seq = read_seqcount_begin(&foo 83 seq = read_seqcount_begin(&foo_seqcount);
84 84
85 /* ... [[read-side critical se 85 /* ... [[read-side critical section]] ... */
86 86
87 } while (read_seqcount_retry(&foo_seqc 87 } while (read_seqcount_retry(&foo_seqcount, seq));
88 88
89 89
90 .. _seqcount_locktype_t: 90 .. _seqcount_locktype_t:
91 91
92 Sequence counters with associated locks (``seq !! 92 Sequence counters with associated locks (``seqcount_LOCKTYPE_t``)
93 ---------------------------------------------- 93 -----------------------------------------------------------------
94 94
95 As discussed at :ref:`seqcount_t`, sequence co 95 As discussed at :ref:`seqcount_t`, sequence count write side critical
96 sections must be serialized and non-preemptibl 96 sections must be serialized and non-preemptible. This variant of
97 sequence counters associate the lock used for 97 sequence counters associate the lock used for writer serialization at
98 initialization time, which enables lockdep to 98 initialization time, which enables lockdep to validate that the write
99 side critical sections are properly serialized 99 side critical sections are properly serialized.
100 100
101 This lock association is a NOOP if lockdep is 101 This lock association is a NOOP if lockdep is disabled and has neither
102 storage nor runtime overhead. If lockdep is en 102 storage nor runtime overhead. If lockdep is enabled, the lock pointer is
103 stored in struct seqcount and lockdep's "lock 103 stored in struct seqcount and lockdep's "lock is held" assertions are
104 injected at the beginning of the write side cr 104 injected at the beginning of the write side critical section to validate
105 that it is properly protected. 105 that it is properly protected.
106 106
107 For lock types which do not implicitly disable 107 For lock types which do not implicitly disable preemption, preemption
108 protection is enforced in the write side funct 108 protection is enforced in the write side function.
109 109
110 The following sequence counters with associate 110 The following sequence counters with associated locks are defined:
111 111
112 - ``seqcount_spinlock_t`` 112 - ``seqcount_spinlock_t``
113 - ``seqcount_raw_spinlock_t`` 113 - ``seqcount_raw_spinlock_t``
114 - ``seqcount_rwlock_t`` 114 - ``seqcount_rwlock_t``
115 - ``seqcount_mutex_t`` 115 - ``seqcount_mutex_t``
116 - ``seqcount_ww_mutex_t`` 116 - ``seqcount_ww_mutex_t``
117 117
118 The sequence counter read and write APIs can t !! 118 The plain seqcount read and write APIs branch out to the specific
119 seqcount_t or any of the seqcount_LOCKNAME_t v !! 119 seqcount_LOCKTYPE_t implementation at compile-time. This avoids kernel
>> 120 API explosion per each new seqcount LOCKTYPE.
120 121
121 Initialization (replace "LOCKNAME" with one of !! 122 Initialization (replace "LOCKTYPE" with one of the supported locks)::
122 123
123 /* dynamic */ 124 /* dynamic */
124 seqcount_LOCKNAME_t foo_seqcount; !! 125 seqcount_LOCKTYPE_t foo_seqcount;
125 seqcount_LOCKNAME_init(&foo_seqcount, !! 126 seqcount_LOCKTYPE_init(&foo_seqcount, &lock);
126 127
127 /* static */ 128 /* static */
128 static seqcount_LOCKNAME_t foo_seqcoun !! 129 static seqcount_LOCKTYPE_t foo_seqcount =
129 SEQCNT_LOCKNAME_ZERO(foo_seqco !! 130 SEQCNT_LOCKTYPE_ZERO(foo_seqcount, &lock);
130 131
131 /* C99 struct init */ 132 /* C99 struct init */
132 struct { 133 struct {
133 .seq = SEQCNT_LOCKNAME_ZERO( !! 134 .seq = SEQCNT_LOCKTYPE_ZERO(foo.seq, &lock),
134 } foo; 135 } foo;
135 136
136 Write path: same as in :ref:`seqcount_t`, whil 137 Write path: same as in :ref:`seqcount_t`, while running from a context
137 with the associated write serialization lock a !! 138 with the associated LOCKTYPE lock acquired.
138 139
139 Read path: same as in :ref:`seqcount_t`. 140 Read path: same as in :ref:`seqcount_t`.
140 <<
141 <<
142 .. _seqcount_latch_t: <<
143 <<
144 Latch sequence counters (``seqcount_latch_t``) <<
145 ---------------------------------------------- <<
146 <<
147 Latch sequence counters are a multiversion con <<
148 where the embedded seqcount_t counter even/odd <<
149 between two copies of protected data. This all <<
150 read path to safely interrupt its own write si <<
151 <<
152 Use seqcount_latch_t when the write side secti <<
153 from interruption by readers. This is typicall <<
154 side can be invoked from NMI handlers. <<
155 <<
156 Check `raw_write_seqcount_latch()` for more in <<
157 <<
158 141
159 .. _seqlock_t: 142 .. _seqlock_t:
160 143
161 Sequential locks (``seqlock_t``) 144 Sequential locks (``seqlock_t``)
162 ================================ 145 ================================
163 146
164 This contains the :ref:`seqcount_t` mechanism 147 This contains the :ref:`seqcount_t` mechanism earlier discussed, plus an
165 embedded spinlock for writer serialization and 148 embedded spinlock for writer serialization and non-preemptibility.
166 149
167 If the read side section can be invoked from h 150 If the read side section can be invoked from hardirq or softirq context,
168 use the write side function variants which dis 151 use the write side function variants which disable interrupts or bottom
169 halves respectively. 152 halves respectively.
170 153
171 Initialization:: 154 Initialization::
172 155
173 /* dynamic */ 156 /* dynamic */
174 seqlock_t foo_seqlock; 157 seqlock_t foo_seqlock;
175 seqlock_init(&foo_seqlock); 158 seqlock_init(&foo_seqlock);
176 159
177 /* static */ 160 /* static */
178 static DEFINE_SEQLOCK(foo_seqlock); 161 static DEFINE_SEQLOCK(foo_seqlock);
179 162
180 /* C99 struct init */ 163 /* C99 struct init */
181 struct { 164 struct {
182 .seql = __SEQLOCK_UNLOCKED(f 165 .seql = __SEQLOCK_UNLOCKED(foo.seql)
183 } foo; 166 } foo;
184 167
185 Write path:: 168 Write path::
186 169
187 write_seqlock(&foo_seqlock); 170 write_seqlock(&foo_seqlock);
188 171
189 /* ... [[write-side critical section]] 172 /* ... [[write-side critical section]] ... */
190 173
191 write_sequnlock(&foo_seqlock); 174 write_sequnlock(&foo_seqlock);
192 175
193 Read path, three categories: 176 Read path, three categories:
194 177
195 1. Normal Sequence readers which never block a 178 1. Normal Sequence readers which never block a writer but they must
196 retry if a writer is in progress by detecti 179 retry if a writer is in progress by detecting change in the sequence
197 number. Writers do not wait for a sequence 180 number. Writers do not wait for a sequence reader::
198 181
199 do { 182 do {
200 seq = read_seqbegin(&foo_seqlo 183 seq = read_seqbegin(&foo_seqlock);
201 184
202 /* ... [[read-side critical se 185 /* ... [[read-side critical section]] ... */
203 186
204 } while (read_seqretry(&foo_seqlock, s 187 } while (read_seqretry(&foo_seqlock, seq));
205 188
206 2. Locking readers which will wait if a writer 189 2. Locking readers which will wait if a writer or another locking reader
207 is in progress. A locking reader in progres 190 is in progress. A locking reader in progress will also block a writer
208 from entering its critical section. This re 191 from entering its critical section. This read lock is
209 exclusive. Unlike rwlock_t, only one lockin 192 exclusive. Unlike rwlock_t, only one locking reader can acquire it::
210 193
211 read_seqlock_excl(&foo_seqlock); 194 read_seqlock_excl(&foo_seqlock);
212 195
213 /* ... [[read-side critical section]] 196 /* ... [[read-side critical section]] ... */
214 197
215 read_sequnlock_excl(&foo_seqlock); 198 read_sequnlock_excl(&foo_seqlock);
216 199
217 3. Conditional lockless reader (as in 1), or l 200 3. Conditional lockless reader (as in 1), or locking reader (as in 2),
218 according to a passed marker. This is used 201 according to a passed marker. This is used to avoid lockless readers
219 starvation (too much retry loops) in case o 202 starvation (too much retry loops) in case of a sharp spike in write
220 activity. First, a lockless read is tried ( 203 activity. First, a lockless read is tried (even marker passed). If
221 that trial fails (odd sequence counter is r 204 that trial fails (odd sequence counter is returned, which is used as
222 the next iteration marker), the lockless re 205 the next iteration marker), the lockless read is transformed to a
223 full locking read and no retry loop is nece 206 full locking read and no retry loop is necessary::
224 207
225 /* marker; even initialization */ 208 /* marker; even initialization */
226 int seq = 0; 209 int seq = 0;
227 do { 210 do {
228 read_seqbegin_or_lock(&foo_seq 211 read_seqbegin_or_lock(&foo_seqlock, &seq);
229 212
230 /* ... [[read-side critical se 213 /* ... [[read-side critical section]] ... */
231 214
232 } while (need_seqretry(&foo_seqlock, s 215 } while (need_seqretry(&foo_seqlock, seq));
233 done_seqretry(&foo_seqlock, seq); 216 done_seqretry(&foo_seqlock, seq);
234 217
235 218
236 API documentation 219 API documentation
237 ================= 220 =================
238 221
239 .. kernel-doc:: include/linux/seqlock.h 222 .. kernel-doc:: include/linux/seqlock.h
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