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47 .Nd kernel shared/exclusive lock
53 .Fn sx_init "struct sx *sx" "const char *description"
55 .Fn sx_destroy "struct sx *sx"
57 .Fn sx_slock "struct sx *sx"
59 .Fn sx_xlock "struct sx *sx"
61 .Fn sx_try_slock "struct sx *sx"
63 .Fn sx_try_xlock "struct sx *sx"
65 .Fn sx_sunlock "struct sx *sx"
67 .Fn sx_xunlock "struct sx *sx"
69 .Fn sx_try_upgrade "struct sx *sx"
71 .Fn sx_downgrade "struct sx *sx"
73 .Fn sx_assert "struct sx *sx" "int what"
75 .Ss Nm Ss utility macros
76 .Fn sx_unlock "struct sx *sx"
77 .Fn SX_SYSINIT "name" "struct sx *sx" "const char *description"
80 .Cd "options INVARIANTS"
81 .Cd "options INVARIANT_SUPPORT"
83 Shared/exclusive locks are used to protect data that are read far more often
84 than they are written.
85 Mutexes are inherently more efficient than shared/exclusive locks, so
86 shared/exclusive locks should be used prudently.
88 Shared/exclusive locks are created with
92 is a pointer to space for a
96 is a pointer to a null-terminated character string that describes the
97 shared/exclusive lock.
98 Shared/exclusive locks are destroyed with
100 Threads acquire and release a shared lock by calling
108 Threads acquire and release an exclusive lock by calling
116 A thread can attempt to upgrade a currently held shared lock to an exclusive
119 A thread that has an exclusive lock can downgrade it to a shared lock by
126 will return 0 if the shared/exclusive lock cannot be acquired immediately;
127 otherwise the shared/exclusive lock will be acquired and a non-zero value will
131 will return 0 if the shared lock cannot be upgraded to an exclusive lock
132 immediately; otherwise the exclusive lock will be acquired and a non-zero value
136 .Cd "options INVARIANTS"
138 .Cd "options INVARIANT_SUPPORT" ,
143 for the assertions specified in
145 and panics if they are not met.
146 The following assertions are supported:
147 .Bl -tag -width ".Dv SX_UNLOCKED"
149 Assert that the current thread has either a shared or an exclusive lock on the
151 lock pointed to by the first argument.
153 Assert that the current thread has a shared lock on the
158 Assert that the current thread has an exclusive lock on the
161 by the first argument.
163 Assert that the current thread has no lock on the
166 by the first argument.
169 For ease of programming,
171 is provided as a macro frontend to the respective functions,
175 Algorithms that are aware of what state the lock is in should use either
176 of the two specific functions for a minor performance benefit.
180 macro is used to generate a call to the
182 routine at system startup in order to initialize a given
185 The parameters are the same as
187 but with an additional argument,
189 that is used in generating unique variable names for the related
190 structures associated with the lock and the sysinit routine.
192 A thread may not hold both a shared lock and an exclusive lock on the same
194 attempting to do so will result in deadlock.
196 A thread may hold a shared or exclusive lock on an
201 lock may not be acquired while holding a mutex.
202 Otherwise, if one thread slept while holding an
204 lock while another thread blocked on the same
206 lock after acquiring a mutex, then the second thread would effectively
207 end up sleeping while holding a mutex, which is not allowed.
215 Currently there is no way to assert that a lock is not held.
216 This is not possible in the
217 .No non- Ns Dv WITNESS
218 case for asserting that this thread
219 does not hold a shared lock.
221 .No non- Ns Dv WITNESS
226 assertions merely check that some thread holds a shared lock.
227 They do not ensure that the current thread holds a shared lock.