1 .\" Copyright (c) 2007 Stephan Uphoff <ups@FreeBSD.org>
2 .\" Copyright (c) 2006 Gleb Smirnoff <glebius@FreeBSD.org>
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28 .\" Based on rwlock.9 man page
46 .Nd kernel reader/writer lock optimized for read-mostly access patterns
52 .Fn rm_init "struct rmlock *rm" "const char *name"
54 .Fn rm_init_flags "struct rmlock *rm" "const char *name" "int opts"
56 .Fn rm_destroy "struct rmlock *rm"
58 .Fn rm_rlock "struct rmlock *rm" "struct rm_priotracker* tracker"
60 .Fn rm_try_rlock "struct rmlock *rm" "struct rm_priotracker* tracker"
62 .Fn rm_wlock "struct rmlock *rm"
64 .Fn rm_runlock "struct rmlock *rm" "struct rm_priotracker* tracker"
66 .Fn rm_wunlock "struct rmlock *rm"
68 .Fn rm_wowned "const struct rmlock *rm"
70 .Fn rm_sleep "void *wchan" "struct rmlock *rm" "int priority" "const char *wmesg" "int timo"
72 .Cd "options INVARIANTS"
73 .Cd "options INVARIANT_SUPPORT"
75 .Fn rm_assert "struct rmlock *rm" "int what"
77 .Fn RM_SYSINIT "name" "struct rmlock *rm" "const char *desc" "int opts"
79 Read-mostly locks allow shared access to protected data by multiple threads,
80 or exclusive access by a single thread.
81 The threads with shared access are known as
83 since they only read the protected data.
84 A thread with exclusive access is known as a
86 since it can modify protected data.
88 Read-mostly locks are designed to be efficient for locks almost exclusively
89 used as reader locks and as such should be used for protecting data that
91 Acquiring an exclusive lock after the lock has been locked for shared access
92 is an expensive operation.
94 Normal read-mostly locks are similar to
96 locks and follow the same lock ordering rules as
99 Read-mostly locks have full priority propagation like mutexes.
102 read-mostly locks propagate priority to both readers and writers.
103 This is implemented via the
105 structure argument supplied to
109 Readers can recurse if the lock is initialized with the
112 however, writers are never allowed to recurse.
114 Sleepable read-mostly locks are created by passing
118 Unlike normal read-mostly locks,
119 sleepable read-mostly locks follow the same lock ordering rules as
122 Sleepable read-mostly locks do not propagate priority to writers,
123 but they do propagate priority to readers.
124 Writers are permitted to sleep while holding a read-mostly lock,
126 Unlike other sleepable locks such as
129 readers must use try operations on other sleepable locks to avoid sleeping.
130 .Ss Macros and Functions
131 .Bl -tag -width indent
132 .It Fn rm_init "struct rmlock *rm" "const char *name"
133 Initialize the read-mostly lock
137 description is used solely for debugging purposes.
138 This function must be called before any other operations
140 .It Fn rm_init_flags "struct rmlock *rm" "const char *name" "int opts"
143 initialize the read-mostly lock
145 with a set of optional flags.
148 arguments contains one or more of the following flags:
149 .Bl -tag -width ".Dv RM_NOWITNESS"
155 Allow threads to recursively acquire shared locks for
158 Create a sleepable read-mostly lock.
160 If the kernel has been compiled with
161 .Cd "option INVARIANTS" ,
165 has not been initialized multiple times without intervening calls to
167 unless this option is specified.
169 .It Fn rm_rlock "struct rmlock *rm" "struct rm_priotracker* tracker"
174 to track read owners of a lock for priority propagation.
175 This data structure is only used internally by
177 and must persist until
180 This data structure can be allocated on the stack since
181 readers cannot sleep.
182 If any thread holds this lock exclusively, the current thread blocks,
183 and its priority is propagated to the exclusive holder.
184 If the lock was initialized with the
188 function can be called when the current thread has already acquired reader
191 .It Fn rm_try_rlock "struct rmlock *rm" "struct rm_priotracker* tracker"
196 will return 0 if the lock cannot be acquired immediately;
198 the lock will be acquired and a non-zero value will be returned.
201 may fail even while the lock is not currently held by a writer.
202 If the lock was initialized with the
206 will succeed if the current thread has already acquired reader access.
207 .It Fn rm_wlock "struct rmlock *rm"
211 If there are any shared owners of the lock, the current thread blocks.
214 function cannot be called recursively.
215 .It Fn rm_runlock "struct rmlock *rm" "struct rm_priotracker* tracker"
216 This function releases a shared lock previously acquired by
220 argument must match the
222 argument used for acquiring the shared lock
223 .It Fn rm_wunlock "struct rmlock *rm"
224 This function releases an exclusive lock previously acquired by
226 .It Fn rm_destroy "struct rmlock *rm"
227 This functions destroys a lock previously initialized with
231 lock must be unlocked.
232 .It Fn rm_wowned "const struct rmlock *rm"
233 This function returns a non-zero value if the current thread owns an
236 .It Fn rm_sleep "void *wchan" "struct rmlock *rm" "int priority" "const char *wmesg" "int timo"
237 This function atomically releases
239 while waiting for an event.
242 lock must be exclusively locked.
243 For more details on the parameters to this function,
246 .It Fn rm_assert "struct rmlock *rm" "int what"
247 This function asserts that the
249 lock is in the state specified by
251 If the assertions are not true and the kernel is compiled with
252 .Cd "options INVARIANTS"
254 .Cd "options INVARIANT_SUPPORT" ,
255 the kernel will panic.
256 Currently the following base assertions are supported:
257 .Bl -tag -width ".Dv RA_UNLOCKED"
259 Assert that current thread holds either a shared or exclusive lock
263 Assert that current thread holds a shared lock of
266 Assert that current thread holds an exclusive lock of
269 Assert that current thread holds neither a shared nor exclusive lock of
273 In addition, one of the following optional flags may be specified with
278 .Bl -tag -width ".Dv RA_NOTRECURSED"
280 Assert that the current thread holds a recursive lock of
282 .It Dv RA_NOTRECURSED
283 Assert that the current thread does not hold a recursive lock of
297 functions appeared in
303 facility was written by
304 .An "Stephan Uphoff" .
305 This manual page was written by
307 for rwlock and modified to reflect rmlock by
308 .An "Stephan Uphoff" .
312 implementation is currently not optimized for single processor systems.
315 can fail transiently even when there is no writer, while another reader
316 updates the state on the local CPU.
320 implementation uses a single per CPU list shared by all
321 rmlocks in the system.
322 If rmlocks become popular, hashing to multiple per CPU queues may
323 be needed to speed up the writer lock process.