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
43 .Nd kernel reader/writer lock optimized for mostly read access patterns
49 .Fn rm_init "struct rmlock *rm" "const char *name"
51 .Fn rm_init_flags "struct rmlock *rm" "const char *name" "int opts"
53 .Fn rm_destroy "struct rmlock *rm"
55 .Fn rm_rlock "struct rmlock *rm" "struct rm_priotracker* tracker"
57 .Fn rm_wlock "struct rmlock *rm"
59 .Fn rm_runlock "struct rmlock *rm" "struct rm_priotracker* tracker"
61 .Fn rm_wunlock "struct rmlock *rm"
63 .Fn rm_wowned "struct rmlock *rm"
65 .Fn RM_SYSINIT "name" "struct rmlock *rm" "const char *desc" "int opts"
67 Mostly reader locks allow shared access to protected data by multiple threads,
68 or exclusive access by a single thread.
69 The threads with shared access are known as
71 since they only read the protected data.
72 A thread with exclusive access is known as a
74 since it can modify protected data.
76 Read mostly locks are designed to be efficient for locks almost exclusively
77 used as reader locks and as such should be used for protecting data that
79 Acquiring an exclusive lock after the lock had been locked for shared access
80 is an expensive operation.
82 Although reader/writer locks look very similar to
84 locks, their usage pattern is different.
85 Reader/writer locks can be treated as mutexes (see
87 with shared/exclusive semantics.
92 can be locked while holding a non-spin mutex, and an
94 cannot be held while sleeping.
97 locks have full priority propagation like mutexes.
100 structure argument supplied in
104 is used to keep track of the read owner(s).
105 Another important property is that shared holders of
107 can recurse if the lock has been initialized with the
109 option, however exclusive locks are not allowed to recurse.
110 .Ss Macros and Functions
111 .Bl -tag -width indent
112 .It Fn rm_init "struct rmlock *rm" "const char *name"
113 Initialize structure located at
115 as mostly reader lock, described by
117 The name description is used solely for debugging purposes.
118 This function must be called before any other operations
120 .It Fn rm_init_flags "struct rmlock *rm" "const char *name" "int opts"
121 Initialize the rm lock just like the
123 function, but specifying a set of optional flags to alter the
129 It contains one or more of the following flags:
130 .Bl -tag -width ".Dv RM_NOWITNESS"
136 Allow threads to recursively acquire exclusive locks for
139 .It Fn rm_rlock "struct rmlock *rm" "struct rm_priotracker* tracker"
145 to track read owners of a lock for priority propagation.
146 This data structure is only used internally by
148 and must persist until
151 This data structure can be allocated on the stack since
152 rmlocks cannot be held while sleeping.
153 If any thread holds this lock exclusively, the current thread blocks,
154 and its priority is propagated to the exclusive holder.
155 If the lock was initialized with the
159 function can be called when the thread has already acquired reader
163 .Dq "recursing on a lock" .
164 .It Fn rm_wlock "struct rmlock *rm"
168 If there are any shared owners of the lock, the current thread blocks.
171 function cannot be called recursively.
172 .It Fn rm_runlock "struct rmlock *rm" "struct rm_priotracker* tracker"
173 This function releases a shared lock previously acquired by
177 argument must match the
179 argument used for acquiring the shared lock
180 .It Fn rm_wunlock "struct rmlock *rm"
181 This function releases an exclusive lock previously acquired by
183 .It Fn rm_destroy "struct rmlock *rm"
184 This functions destroys a lock previously initialized with
188 lock must be unlocked.
189 .It Fn rm_wowned "struct rmlock *rm"
190 This function returns a non-zero value if the current thread owns an
203 functions appeared in
209 facility was written by
210 .An "Stephan Uphoff" .
211 This manual page was written by
213 for rwlock and modified to reflect rmlock by
214 .An "Stephan Uphoff" .
218 implementation is currently not optimized for single processor systems.
222 implementation uses a single per CPU list shared by all
223 rmlocks in the system.
224 If rmlocks become popular, hashing to multiple per CPU queues may
225 be needed to speed up the writer lock process.
229 can currently not be used as a lock argument for condition variable