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41 .Nd kernel reader/writer lock
47 .Fn rw_init "struct rwlock *rw" "const char *name"
49 .Fn rw_rlock "struct rwlock *rw"
51 .Fn rw_wlock "struct rwlock *rw"
53 .Fn rw_runlock "struct rwlock *rw"
55 .Fn rw_wunlock "struct rwlock *rw"
57 .Fn rw_try_upgrade "struct rwlock *rw"
59 .Fn rw_downgrade "struct rwlock *rw"
61 .Fn rw_initialized "struct rwlock *rw"
63 .Fn rw_destroy "struct rwlock *rw"
65 .Cd "options INVARIANTS"
66 .Cd "options INVARIANT_SUPPORT"
68 .Fn rw_assert "struct rwlock *rw" "int what"
70 .Fn RW_SYSINIT "name" "struct rwlock *rw" "const char *desc"
72 Reader/writer locks allow shared access to protected data by multiple threads,
73 or exclusive access by a single thread.
74 The threads with shared access are known as
76 since they only read the protected data.
77 A thread with exclusive access is known as a
79 since it can modify protected data.
81 Although reader/writer locks look very similar to
83 locks, their usage pattern is different.
84 Reader/writer locks can be treated as mutexes (see
86 with shared/exclusive semantics.
91 can be locked while holding a non-spin mutex, and an
93 cannot be held while sleeping.
96 locks have priority propagation like mutexes, but priority
97 can be propagated only to an exclusive holder.
98 This limitation comes from the fact that shared owners
100 Another important property is that shared holders of
103 but exclusive locks are not allowed to recurse.
104 .Ss Macros and Functions
105 .Bl -tag -width indent
106 .It Fn rw_init "struct rwlock *rw" "const char *name"
107 Initialize structure located at
109 as reader/writer lock, described by name
111 The description is used solely for debugging purposes.
112 This function must be called before any other operations
114 .It Fn rw_rlock "struct rwlock *rw"
118 If any thread holds this lock exclusively, the current thread blocks,
119 and its priority is propagated to the exclusive holder.
122 function can be called when the thread has already acquired reader
126 .Dq "recursing on a lock" .
127 .It Fn rw_wlock "struct rwlock *rw"
131 If there are any shared owners of the lock, the current thread blocks.
134 function cannot be called recursively.
135 .It Fn rw_runlock "struct rwlock *rw"
136 This function releases a shared lock previously acquired by
138 .It Fn rw_wunlock "struct rwlock *rw"
139 This function releases an exclusive lock previously acquired by
141 .It Fn rw_try_upgrade "struct rwlock *rw"
142 Attempt to upgrade a single shared lock to an exclusive lock.
143 The current thread must hold a shared lock of
145 This will only succeed if the current thread holds the only shared lock on
147 and it only holds a single shared lock.
148 If the attempt succeeds
150 will return a non-zero value,
151 and the current thread will hold an exclusive lock.
155 and the current thread will still hold a shared lock.
156 .It Fn rw_downgrade "struct rwlock *rw"
157 Convert an exclusive lock into a single shared lock.
158 The current thread must hold an exclusive lock of
160 .It Fn rw_initialized "struct rwlock *rw"
161 This function returns non-zero if
163 has been initialized, and zero otherwise.
164 .It Fn rw_destroy "struct rwlock *rw"
165 This functions destroys a lock previously initialized with
169 lock must be unlocked.
170 .It Fn rw_assert "struct rwlock *rw" "int what"
171 This function allows assertions specified in
175 If the assertions are not true and the kernel is compiled
177 .Cd "options INVARIANTS"
179 .Cd "options INVARIANT_SUPPORT" ,
180 the kernel will panic.
181 Currently the following assertions are supported:
182 .Bl -tag -width ".Dv RA_UNLOCKED"
184 Assert that current thread holds either a shared or exclusive lock
188 Assert that current thread holds a shared lock of
191 Assert that current thread holds an exclusive lock of
194 Assert that current thread holds neither a shared nor exclusive lock of
205 functions appeared in
211 facility was written by
213 This manual page was written by
214 .An "Gleb Smirnoff" .
218 is not included in the kernel,
219 then it is impossible to assert that the current thread does or does not
227 assertions merely check that some thread holds a shared lock.
229 Reader/writer is a bit of an awkward name.