1======================================
2Sequence counters and sequential locks
3======================================
4
5Introduction
6============
7
8Sequence counters are a reader-writer consistency mechanism with
9lockless readers (read-only retry loops), and no writer starvation. They
10are used for data that's rarely written to (e.g. system time), where the
11reader wants a consistent set of information and is willing to retry if
12that information changes.
13
14A data set is consistent when the sequence count at the beginning of the
15read side critical section is even and the same sequence count value is
16read again at the end of the critical section. The data in the set must
17be copied out inside the read side critical section. If the sequence
18count has changed between the start and the end of the critical section,
19the reader must retry.
20
21Writers increment the sequence count at the start and the end of their
22critical section. After starting the critical section the sequence count
23is odd and indicates to the readers that an update is in progress. At
24the end of the write side critical section the sequence count becomes
25even again which lets readers make progress.
26
27A sequence counter write side critical section must never be preempted
28or interrupted by read side sections. Otherwise the reader will spin for
29the entire scheduler tick due to the odd sequence count value and the
30interrupted writer. If that reader belongs to a real-time scheduling
31class, it can spin forever and the kernel will livelock.
32
33This mechanism cannot be used if the protected data contains pointers,
34as the writer can invalidate a pointer that the reader is following.
35
36
37.. _seqcount_t:
38
39Sequence counters (``seqcount_t``)
40==================================
41
42This is the raw counting mechanism, which does not protect against
43multiple writers.  Write side critical sections must thus be serialized
44by an external lock.
45
46If the write serialization primitive is not implicitly disabling
47preemption, preemption must be explicitly disabled before entering the
48write side section. If the read section can be invoked from hardirq or
49softirq contexts, interrupts or bottom halves must also be respectively
50disabled before entering the write section.
51
52If it's desired to automatically handle the sequence counter
53requirements of writer serialization and non-preemptibility, use
54:ref:`seqlock_t` instead.
55
56Initialization::
57
58	/* dynamic */
59	seqcount_t foo_seqcount;
60	seqcount_init(&foo_seqcount);
61
62	/* static */
63	static seqcount_t foo_seqcount = SEQCNT_ZERO(foo_seqcount);
64
65	/* C99 struct init */
66	struct {
67		.seq   = SEQCNT_ZERO(foo.seq),
68	} foo;
69
70Write path::
71
72	/* Serialized context with disabled preemption */
73
74	write_seqcount_begin(&foo_seqcount);
75
76	/* ... [[write-side critical section]] ... */
77
78	write_seqcount_end(&foo_seqcount);
79
80Read path::
81
82	do {
83		seq = read_seqcount_begin(&foo_seqcount);
84
85		/* ... [[read-side critical section]] ... */
86
87	} while (read_seqcount_retry(&foo_seqcount, seq));
88
89
90.. _seqcount_locktype_t:
91
92Sequence counters with associated locks (``seqcount_LOCKNAME_t``)
93-----------------------------------------------------------------
94
95As discussed at :ref:`seqcount_t`, sequence count write side critical
96sections must be serialized and non-preemptible. This variant of
97sequence counters associate the lock used for writer serialization at
98initialization time, which enables lockdep to validate that the write
99side critical sections are properly serialized.
100
101This lock association is a NOOP if lockdep is disabled and has neither
102storage nor runtime overhead. If lockdep is enabled, the lock pointer is
103stored in struct seqcount and lockdep's "lock is held" assertions are
104injected at the beginning of the write side critical section to validate
105that it is properly protected.
106
107For lock types which do not implicitly disable preemption, preemption
108protection is enforced in the write side function.
109
110The following sequence counters with associated locks are defined:
111
112  - ``seqcount_spinlock_t``
113  - ``seqcount_raw_spinlock_t``
114  - ``seqcount_rwlock_t``
115  - ``seqcount_mutex_t``
116  - ``seqcount_ww_mutex_t``
117
118The sequence counter read and write APIs can take either a plain
119seqcount_t or any of the seqcount_LOCKNAME_t variants above.
120
121Initialization (replace "LOCKNAME" with one of the supported locks)::
122
123	/* dynamic */
124	seqcount_LOCKNAME_t foo_seqcount;
125	seqcount_LOCKNAME_init(&foo_seqcount, &lock);
126
127	/* static */
128	static seqcount_LOCKNAME_t foo_seqcount =
129		SEQCNT_LOCKNAME_ZERO(foo_seqcount, &lock);
130
131	/* C99 struct init */
132	struct {
133		.seq   = SEQCNT_LOCKNAME_ZERO(foo.seq, &lock),
134	} foo;
135
136Write path: same as in :ref:`seqcount_t`, while running from a context
137with the associated write serialization lock acquired.
138
139Read path: same as in :ref:`seqcount_t`.
140
141
142.. _seqcount_latch_t:
143
144Latch sequence counters (``seqcount_latch_t``)
145----------------------------------------------
146
147Latch sequence counters are a multiversion concurrency control mechanism
148where the embedded seqcount_t counter even/odd value is used to switch
149between two copies of protected data. This allows the sequence counter
150read path to safely interrupt its own write side critical section.
151
152Use seqcount_latch_t when the write side sections cannot be protected
153from interruption by readers. This is typically the case when the read
154side can be invoked from NMI handlers.
155
156Check `write_seqcount_latch()` for more information.
157
158
159.. _seqlock_t:
160
161Sequential locks (``seqlock_t``)
162================================
163
164This contains the :ref:`seqcount_t` mechanism earlier discussed, plus an
165embedded spinlock for writer serialization and non-preemptibility.
166
167If the read side section can be invoked from hardirq or softirq context,
168use the write side function variants which disable interrupts or bottom
169halves respectively.
170
171Initialization::
172
173	/* dynamic */
174	seqlock_t foo_seqlock;
175	seqlock_init(&foo_seqlock);
176
177	/* static */
178	static DEFINE_SEQLOCK(foo_seqlock);
179
180	/* C99 struct init */
181	struct {
182		.seql   = __SEQLOCK_UNLOCKED(foo.seql)
183	} foo;
184
185Write path::
186
187	write_seqlock(&foo_seqlock);
188
189	/* ... [[write-side critical section]] ... */
190
191	write_sequnlock(&foo_seqlock);
192
193Read path, three categories:
194
1951. Normal Sequence readers which never block a writer but they must
196   retry if a writer is in progress by detecting change in the sequence
197   number.  Writers do not wait for a sequence reader::
198
199	do {
200		seq = read_seqbegin(&foo_seqlock);
201
202		/* ... [[read-side critical section]] ... */
203
204	} while (read_seqretry(&foo_seqlock, seq));
205
2062. Locking readers which will wait if a writer or another locking reader
207   is in progress. A locking reader in progress will also block a writer
208   from entering its critical section. This read lock is
209   exclusive. Unlike rwlock_t, only one locking reader can acquire it::
210
211	read_seqlock_excl(&foo_seqlock);
212
213	/* ... [[read-side critical section]] ... */
214
215	read_sequnlock_excl(&foo_seqlock);
216
2173. Conditional lockless reader (as in 1), or locking reader (as in 2),
218   according to a passed marker. This is used to avoid lockless readers
219   starvation (too much retry loops) in case of a sharp spike in write
220   activity. First, a lockless read is tried (even marker passed). If
221   that trial fails (odd sequence counter is returned, which is used as
222   the next iteration marker), the lockless read is transformed to a
223   full locking read and no retry loop is necessary::
224
225	/* marker; even initialization */
226	int seq = 0;
227	do {
228		read_seqbegin_or_lock(&foo_seqlock, &seq);
229
230		/* ... [[read-side critical section]] ... */
231
232	} while (need_seqretry(&foo_seqlock, seq));
233	done_seqretry(&foo_seqlock, seq);
234
235
236API documentation
237=================
238
239.. kernel-doc:: include/linux/seqlock.h
240