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27 .Dd April 1, 2008
28 .Dt RWLOCK 9
29 .Os
30 .Sh NAME
31 .Nm rwlock ,
32 .Nm rw_init ,
33 .Nm rw_init_flags,
34 .Nm rw_destroy ,
35 .Nm rw_rlock ,
36 .Nm rw_wlock ,
37 .Nm rw_runlock ,
38 .Nm rw_wunlock ,
39 .Nm rw_unlock ,
40 .Nm rw_try_rlock ,
41 .Nm rw_try_upgrade ,
42 .Nm rw_try_wlock ,
43 .Nm rw_downgrade ,
44 .Nm rw_sleep ,
45 .Nm rw_initialized ,
46 .Nm rw_wowned ,
47 .Nm rw_assert ,
48 .Nm RW_SYSINIT
49 .Nd kernel reader/writer lock
50 .Sh SYNOPSIS
51 .In sys/param.h
52 .In sys/lock.h
53 .In sys/rwlock.h
54 .Ft void
55 .Fn rw_init "struct rwlock *rw" "const char *name"
56 .Ft void
57 .Fn rw_init_flags "struct rwlock *rw" "const char *name" "int opts"
58 .Ft void
59 .Fn rw_destroy "struct rwlock *rw"
60 .Ft void
61 .Fn rw_rlock "struct rwlock *rw"
62 .Ft void
63 .Fn rw_wlock "struct rwlock *rw"
64 .Ft int
65 .Fn rw_try_rlock "struct rwlock *rw"
66 .Ft int
67 .Fn rw_try_wlock "struct rwlock *rw"
68 .Ft void
69 .Fn rw_runlock "struct rwlock *rw"
70 .Ft void
71 .Fn rw_wunlock "struct rwlock *rw"
72 .Ft void
73 .Fn rw_unlock "struct rwlock *rw"
74 .Ft int
75 .Fn rw_try_upgrade "struct rwlock *rw"
76 .Ft void
77 .Fn rw_downgrade "struct rwlock *rw"
78 .Ft int
79 .Fn rw_sleep "void *chan" "struct rwlock *rw" "int priority" "const char *wmesg" "int timo"
80 .Ft int
81 .Fn rw_initialized "struct rwlock *rw"
82 .Ft int
83 .Fn rw_wowned "struct rwlock *rw"
84 .Pp
85 .Cd "options INVARIANTS"
86 .Cd "options INVARIANT_SUPPORT"
87 .Ft void
88 .Fn rw_assert "struct rwlock *rw" "int what"
89 .In sys/kernel.h
90 .Fn RW_SYSINIT "name" "struct rwlock *rw" "const char *desc"
91 .Sh DESCRIPTION
92 Reader/writer locks allow shared access to protected data by multiple threads,
93 or exclusive access by a single thread.
94 The threads with shared access are known as
95 .Em readers
96 since they only read the protected data.
97 A thread with exclusive access is known as a
98 .Em writer
99 since it can modify protected data.
100 .Pp
101 Although reader/writer locks look very similar to
102 .Xr sx 9
103 locks, their usage pattern is different.
104 Reader/writer locks can be treated as mutexes (see
105 .Xr mutex 9 )
106 with shared/exclusive semantics.
107 Unlike
108 .Xr sx 9 ,
109 an
110 .Nm
111 can be locked while holding a non-spin mutex, and an
112 .Nm
113 cannot be held while sleeping.
114 The
115 .Nm
116 locks have priority propagation like mutexes, but priority
117 can be propagated only to an exclusive holder.
118 This limitation comes from the fact that shared owners
119 are anonymous.
120 Another important property is that shared holders of
121 .Nm
122 can recurse,
123 and exclusive locks can be made recursive selectively.
124 .Ss Macros and Functions
125 .Bl -tag -width indent
126 .It Fn rw_init "struct rwlock *rw" "const char *name"
127 Initialize structure located at
128 .Fa rw
129 as reader/writer lock, described by name
130 .Fa name .
131 The description is used solely for debugging purposes.
132 This function must be called before any other operations
133 on the lock.
134 .It Fn rw_init_flags "struct rwlock *rw" "const char *name" "int opts"
135 Initialize the rw lock just like the
136 .Fn rw_init
137 function, but specifying a set of optional flags to alter the
138 behaviour of
139 .Fa rw ,
140 through the
141 .Fa opts
142 argument.
143 It contains one or more of the following flags:
144 .Bl -tag -width ".Dv RW_NOPROFILE"
145 .It Dv RW_DUPOK
146 Witness should not log messages about duplicate locks being acquired.
147 .It Dv RW_NOPROFILE
148 Do not profile this lock.
149 .It Dv RW_NOWITNESS
150 Instruct
151 .Xr witness 4
152 to ignore this lock.
153 .It Dv RW_QUIET
154 Do not log any operations for this lock via
155 .Xr ktr 4 .
156 .It Dv RW_RECURSE
157 Allow threads to recursively acquire exclusive locks for
158 .Fa rw .
159 .El
160 .It Fn rw_rlock "struct rwlock *rw"
161 Lock
162 .Fa rw
163 as a reader.
164 If any thread holds this lock exclusively, the current thread blocks,
165 and its priority is propagated to the exclusive holder.
166 The
167 .Fn rw_rlock
168 function can be called when the thread has already acquired reader
169 access on
170 .Fa rw .
171 This is called
172 .Dq "recursing on a lock" .
173 .It Fn rw_wlock "struct rwlock *rw"
174 Lock
175 .Fa rw
176 as a writer.
177 If there are any shared owners of the lock, the current thread blocks.
178 The
179 .Fn rw_wlock
180 function can be called recursively only if
181 .Fa rw
182 has been initialized with the
183 .Dv RW_RECURSE
184 option enabled.
185 .It Fn rw_try_rlock "struct rwlock *rw"
186 Try to lock
187 .Fa rw
188 as a reader.
189 This function will return true if the operation succeeds, otherwise 0
190 will be returned.
191 .It Fn rw_try_wlock "struct rwlock *rw"
192 Try to lock
193 .Fa rw
194 as a writer.
195 This function will return true if the operation succeeds, otherwise 0
196 will be returned.
197 .It Fn rw_runlock "struct rwlock *rw"
198 This function releases a shared lock previously acquired by
199 .Fn rw_rlock .
200 .It Fn rw_wunlock "struct rwlock *rw"
201 This function releases an exclusive lock previously acquired by
202 .Fn rw_wlock .
203 .It Fn rw_unlock "struct rwlock *rw"
204 This function releases a shared lock previously acquired by
205 .Fn rw_rlock
206 or an exclusive lock previously acquired by
207 .Fn rw_wlock .
208 .It Fn rw_try_upgrade "struct rwlock *rw"
209 Attempt to upgrade a single shared lock to an exclusive lock.
210 The current thread must hold a shared lock of
211 .Fa rw .
212 This will only succeed if the current thread holds the only shared lock on
213 .Fa rw ,
214 and it only holds a single shared lock.
215 If the attempt succeeds
216 .Fn rw_try_upgrade
217 will return a non-zero value,
218 and the current thread will hold an exclusive lock.
219 If the attempt fails
220 .Fn rw_try_upgrade
221 will return zero,
222 and the current thread will still hold a shared lock.
223 .It Fn rw_downgrade "struct rwlock *rw"
224 Convert an exclusive lock into a single shared lock.
225 The current thread must hold an exclusive lock of
226 .Fa rw .
227 .It Fn rw_sleep "void *chan" "struct rwlock *rw" "int priority" "const char *wmesg" "int timo"
228 Atomically release
229 .Fa rw
230 while waiting for an event.
231 For more details on the parameters to this function,
232 see
233 .Xr sleep 9 .
234 .It Fn rw_initialized "struct rwlock *rw"
235 This function returns non-zero if
236 .Fa rw
237 has been initialized, and zero otherwise.
238 .It Fn rw_destroy "struct rwlock *rw"
239 This functions destroys a lock previously initialized with
240 .Fn rw_init .
241 The
242 .Fa rw
243 lock must be unlocked.
244 .It Fn rw_wowned "struct rwlock *rw"
245 This function returns a non-zero value if the current thread owns an
246 exclusive lock on
247 .Fa rw .
248 .It Fn rw_assert "struct rwlock *rw" "int what"
249 This function allows assertions specified in
250 .Fa what
251 to be made about
252 .Fa rw .
253 If the assertions are not true and the kernel is compiled
254 with
255 .Cd "options INVARIANTS"
256 and
257 .Cd "options INVARIANT_SUPPORT" ,
258 the kernel will panic.
259 Currently the following assertions are supported:
260 .Bl -tag -width ".Dv RA_UNLOCKED"
261 .It Dv RA_LOCKED
262 Assert that current thread holds either a shared or exclusive lock
263 of
264 .Fa rw .
265 .It Dv RA_RLOCKED
266 Assert that current thread holds a shared lock of
267 .Fa rw .
268 .It Dv RA_WLOCKED
269 Assert that current thread holds an exclusive lock of
270 .Fa rw .
271 .It Dv RA_UNLOCKED
272 Assert that current thread holds neither a shared nor exclusive lock of
273 .Fa rw .
274 .El
275 .El
276 .Sh SEE ALSO
277 .Xr locking 9 ,
278 .Xr mutex 9 ,
279 .Xr panic 9 ,
280 .Xr sema 9 ,
281 .Xr sx 9
282 .Sh HISTORY
283 These
284 functions appeared in
285 .Fx 7.0 .
286 .Sh AUTHORS
287 .An -nosplit
288 The
289 .Nm
290 facility was written by
291 .An "John Baldwin" .
292 This manual page was written by
293 .An "Gleb Smirnoff" .
294 .Sh BUGS
295 If
296 .Dv WITNESS
297 is not included in the kernel,
298 then it is impossible to assert that the current thread does or does not
299 hold a read lock.
300 In the
301 .Pf non- Dv WITNESS
302 case, the
303 .Dv RA_LOCKED
304 and
305 .Dv RA_RLOCKED
306 assertions merely check that some thread holds a read lock.
307 .Pp
308 Reader/writer is a bit of an awkward name.
309 An
310 .Nm
311 can also be called a
312 .Dq Robert Watson
313 lock if desired.