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54 .Nd kernel shared/exclusive lock
60 .Fn sx_init "struct sx *sx" "const char *description"
62 .Fn sx_init_flags "struct sx *sx" "const char *description" "int opts"
64 .Fn sx_destroy "struct sx *sx"
66 .Fn sx_slock "struct sx *sx"
68 .Fn sx_xlock "struct sx *sx"
70 .Fn sx_slock_sig "struct sx *sx"
72 .Fn sx_xlock_sig "struct sx *sx"
74 .Fn sx_try_slock "struct sx *sx"
76 .Fn sx_try_xlock "struct sx *sx"
78 .Fn sx_sunlock "struct sx *sx"
80 .Fn sx_xunlock "struct sx *sx"
82 .Fn sx_unlock "struct sx *sx"
84 .Fn sx_try_upgrade "struct sx *sx"
86 .Fn sx_downgrade "struct sx *sx"
88 .Fn sx_sleep "void *chan" "struct sx *sx" "int priority" "const char *wmesg" "int timo"
90 .Fn sx_xholder "struct sx *sx"
92 .Fn sx_xlocked "const struct sx *sx"
94 .Cd "options INVARIANTS"
95 .Cd "options INVARIANT_SUPPORT"
97 .Fn sx_assert "const struct sx *sx" "int what"
99 .Fn SX_SYSINIT "name" "struct sx *sx" "const char *desc"
100 .Fn SX_SYSINIT_FLAGS "name" "struct sx *sx" "const char *desc" "int flags"
102 Shared/exclusive locks are used to protect data that are read far more often
103 than they are written.
104 Shared/exclusive locks do not implement priority propagation like mutexes and
105 reader/writer locks to prevent priority inversions, so
106 shared/exclusive locks should be used prudently.
108 Shared/exclusive locks are created with either
114 is a pointer to space for a
118 is a pointer to a null-terminated character string that describes the
119 shared/exclusive lock.
124 specifies a set of optional flags to alter the behavior of
126 It contains one or more of the following flags:
127 .Bl -tag -width SX_NOWITNESS
129 Witness should not log messages about duplicate locks being acquired.
135 Do not profile this lock.
137 Allow threads to recursively acquire exclusive locks for
140 Do not log any operations for this lock via
143 If the kernel has been compiled with
144 .Cd "options INVARIANTS" ,
148 has not been initialized multiple times without intervening calls to
150 unless this option is specified.
153 Shared/exclusive locks are destroyed with
157 must not be locked by any thread when it is destroyed.
159 Threads acquire and release a shared lock by calling
168 Threads acquire and release an exclusive lock by calling
177 A thread can attempt to upgrade a currently held shared lock to an exclusive
180 A thread that has an exclusive lock can downgrade it to a shared lock by
187 will return 0 if the shared/exclusive lock cannot be acquired immediately;
188 otherwise the shared/exclusive lock will be acquired and a non-zero value will
192 will return 0 if the shared lock cannot be upgraded to an exclusive lock
193 immediately; otherwise the exclusive lock will be acquired and a non-zero value
199 do the same as their normal versions but performing an interruptible sleep.
200 They return a non-zero value if the sleep has been interrupted by a signal
201 or an interrupt, otherwise 0.
203 A thread can atomically release a shared/exclusive lock while waiting for an
206 For more details on the parameters to this function,
211 .Cd "options INVARIANTS"
213 .Cd "options INVARIANT_SUPPORT" ,
218 for the assertions specified in
220 and panics if they are not met.
221 One of the following assertions must be specified:
222 .Bl -tag -width ".Dv SA_UNLOCKED"
224 Assert that the current thread has either a shared or an exclusive lock on the
226 lock pointed to by the first argument.
228 Assert that the current thread has a shared lock on the
233 Assert that the current thread has an exclusive lock on the
236 by the first argument.
238 Assert that the current thread has no lock on the
241 by the first argument.
244 In addition, one of the following optional assertions may be included with
251 .Bl -tag -width ".Dv SA_NOTRECURSED"
253 Assert that the current thread has a recursed lock on
255 .It Dv SA_NOTRECURSED
256 Assert that the current thread does not have a recursed lock on
261 will return a pointer to the thread which currently holds an exclusive lock on
263 If no thread holds an exclusive lock on
270 will return non-zero if the current thread holds the exclusive lock;
271 otherwise, it will return zero.
273 For ease of programming,
275 is provided as a macro frontend to the respective functions,
279 Algorithms that are aware of what state the lock is in should use either
280 of the two specific functions for a minor performance benefit.
284 macro is used to generate a call to the
286 routine at system startup in order to initialize a given
289 The parameters are the same as
291 but with an additional argument,
293 that is used in generating unique variable names for the related
294 structures associated with the lock and the sysinit routine.
297 macro can similarly be used to initialize a given
302 A thread may not hold both a shared lock and an exclusive lock on the same
304 attempting to do so will result in deadlock.
306 A thread may hold a shared or exclusive lock on an
311 lock may not be acquired while holding a mutex.
312 Otherwise, if one thread slept while holding an
314 lock while another thread blocked on the same
316 lock after acquiring a mutex, then the second thread would effectively
317 end up sleeping while holding a mutex, which is not allowed.
328 cannot assert whether the current thread does or does not hold a shared lock.
334 thread holds a shared lock.
335 They cannot ensure that the current thread holds a shared lock.
338 can only assert that the current thread does not hold an exclusive lock.