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
42 .Nd mostly read lock - a kernel reader/writer lock optimized for mostly read access patterns
48 .Fn rm_init "struct rmlock *rm" "const char *name" "int opts"
50 .Fn rm_destroy "struct rmlock *rm"
52 .Fn rm_rlock "struct rmlock *rm" "struct rm_priotracker* tracker"
54 .Fn rm_wlock "struct rmlock *rm"
56 .Fn rm_runlock "struct rmlock *rm" "struct rm_priotracker* tracker"
58 .Fn rm_wunlock "struct rmlock *rm"
60 .Fn rm_wowned "struct rmlock *rm"
62 .Fn RM_SYSINIT "name" "struct rmlock *rm" "const char *desc" "int opts"
64 Mostly reader locks allow shared access to protected data by multiple threads,
65 or exclusive access by a single thread.
66 The threads with shared access are known as
68 since they only read the protected data.
69 A thread with exclusive access is known as a
71 since it can modify protected data.
73 Read mostly locks are designed to be efficient for locks almost exclusively
74 used as reader locks and as such should be used for protecting data that
76 Acquiring an exclusive lock after the lock had been locked for shared access
77 is an expensive operation.
79 Although reader/writer locks look very similar to
81 locks, their usage pattern is different.
82 Reader/writer locks can be treated as mutexes (see
84 with shared/exclusive semantics.
89 can be locked while holding a non-spin mutex, and an
91 cannot be held while sleeping.
94 locks have full priority propagation like mutexes. The
96 structure argument supplied in
100 is used to keep track of the read owner(s).
101 Another important property is that shared holders of
103 can recurse if the lock has been initialized with the
105 option, however exclusive locks are not allowed to recurse.
106 .Ss Macros and Functions
107 .Bl -tag -width indent
108 .It Fn rm_init "struct rmlock *rm" "const char *name" "int opts"
109 Initialize structure located at
111 as mostly reader lock, described by
113 Optionally allowing readers to recurse by setting
117 The name description is used solely for debugging purposes.
118 This function must be called before any other operations
120 .It Fn rm_rlock "struct rmlock *rm" "struct rm_priotracker* tracker"
125 to track read owners of a lock for priority propagation.
126 This data structure is only used internally by
128 and must persist until rm_runlock has been called.
129 This data structure can be allocated on the stack since
130 rmlocks cannot be held while sleeping.
131 If any thread holds this lock exclusively, the current thread blocks,
132 and its priority is propagated to the exclusive holder.
133 If the lock was initialized with the
137 function can be called when the thread has already acquired reader
141 .Dq "recursing on a lock" .
142 .It Fn rm_wlock "struct rmlock *rm"
146 If there are any shared owners of the lock, the current thread blocks.
149 function cannot be called recursively.
150 .It Fn rm_runlock "struct rmlock *rm" "struct rm_priotracker* tracker"
151 This function releases a shared lock previously acquired by
155 argument must match the
157 argument used for acquiring the shared lock
158 .It Fn rm_wunlock "struct rmlock *rm"
159 This function releases an exclusive lock previously acquired by
161 .It Fn rm_destroy "struct rmlock *rm"
162 This functions destroys a lock previously initialized with
166 lock must be unlocked.
167 .It Fn rm_wowned "struct rmlock *rm"
168 This function returns a non-zero value if the current thread owns an
182 functions appeared in
188 facility was written by
189 .An "Stephan Uphoff" .
190 This manual page was written by
192 for rwlock and modified to reflect rmlock by
193 .An "Stephan Uphoff" .
198 implementation is currently not optimized for single processor systems.
202 implementation uses a single per cpu list shared by all
203 rmlocks in the system.
204 If rmlocks become popular, hashing to multiple per cpu queues may
205 be needed to speed up the writer lock process.
209 can currently not be used as a lock argument for condition variable