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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:
128 Witness should not log messages about duplicate locks being acquired.
134 Do not profile this lock.
136 Allow threads to recursively acquire exclusive locks for
139 Do not log any operations for this lock via
142 If the kernel has been compiled with
143 .Cd "options INVARIANTS" ,
147 has not been initialized multiple times without intervening calls to
149 unless this option is specified.
152 Shared/exclusive locks are destroyed with
156 must not be locked by any thread when it is destroyed.
158 Threads acquire and release a shared lock by calling
167 Threads acquire and release an exclusive lock by calling
176 A thread can attempt to upgrade a currently held shared lock to an exclusive
179 A thread that has an exclusive lock can downgrade it to a shared lock by
186 will return 0 if the shared/exclusive lock cannot be acquired immediately;
187 otherwise the shared/exclusive lock will be acquired and a non-zero value will
191 will return 0 if the shared lock cannot be upgraded to an exclusive lock
192 immediately; otherwise the exclusive lock will be acquired and a non-zero value
198 do the same as their normal versions but performing an interruptible sleep.
199 They return a non-zero value if the sleep has been interrupted by a signal
200 or an interrupt, otherwise 0.
202 A thread can atomically release a shared/exclusive lock while waiting for an
205 For more details on the parameters to this function,
210 .Cd "options INVARIANTS"
212 .Cd "options INVARIANT_SUPPORT" ,
217 for the assertions specified in
219 and panics if they are not met.
220 One of the following assertions must be specified:
221 .Bl -tag -width ".Dv SA_UNLOCKED"
223 Assert that the current thread has either a shared or an exclusive lock on the
225 lock pointed to by the first argument.
227 Assert that the current thread has a shared lock on the
232 Assert that the current thread has an exclusive lock on the
235 by the first argument.
237 Assert that the current thread has no lock on the
240 by the first argument.
243 In addition, one of the following optional assertions may be included with
250 .Bl -tag -width ".Dv SA_NOTRECURSED"
252 Assert that the current thread has a recursed lock on
254 .It Dv SA_NOTRECURSED
255 Assert that the current thread does not have a recursed lock on
260 will return a pointer to the thread which currently holds an exclusive lock on
262 If no thread holds an exclusive lock on
269 will return non-zero if the current thread holds the exclusive lock;
270 otherwise, it will return zero.
272 For ease of programming,
274 is provided as a macro frontend to the respective functions,
278 Algorithms that are aware of what state the lock is in should use either
279 of the two specific functions for a minor performance benefit.
283 macro is used to generate a call to the
285 routine at system startup in order to initialize a given
288 The parameters are the same as
290 but with an additional argument,
292 that is used in generating unique variable names for the related
293 structures associated with the lock and the sysinit routine.
296 macro can similarly be used to initialize a given
301 A thread may not hold both a shared lock and an exclusive lock on the same
303 attempting to do so will result in deadlock.
305 A thread may hold a shared or exclusive lock on an
310 lock may not be acquired while holding a mutex.
311 Otherwise, if one thread slept while holding an
313 lock while another thread blocked on the same
315 lock after acquiring a mutex, then the second thread would effectively
316 end up sleeping while holding a mutex, which is not allowed.
327 cannot assert whether the current thread does or does not hold a shared lock.
333 thread holds a shared lock.
334 They cannot ensure that the current thread holds a shared lock.
337 can only assert that the current thread does not hold an exclusive lock.