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SX(9) BSD Kernel Developer's Manual SX(9) NAME sx, sx_init, sx_destroy, sx_slock, sx_xlock, sx_try_slock, sx_try_xlock, sx_sunlock, sx_xunlock, sx_try_upgrade, sx_downgrade, sx_assert, SX_SYSINIT -- kernel shared/exclusive lock SYNOPSIS #include <sys/param.h> #include <sys/lock.h> #include <sys/mutex.h> #include <sys/sx.h> void sx_init(struct sx *sx, const char *description); void sx_destroy(struct sx *sx); void sx_slock(struct sx *sx); void sx_xlock(struct sx *sx); int sx_try_slock(struct sx *sx); int sx_try_xlock(struct sx *sx); void sx_sunlock(struct sx *sx); void sx_xunlock(struct sx *sx); int sx_try_upgrade(struct sx *sx); void sx_downgrade(struct sx *sx); void sx_assert(struct sx *sx, int what); SX_SYSINIT(name, struct sx *sx, const char *description); DESCRIPTION Shared/exclusive locks are used to protect data that are read far more often than they are written. Mutexes are inherently more efficient than shared/exclusive locks, so shared/exclusive locks should be used pru- dently. Shared/exclusive locks are created with sx_init(), where sx is a pointer to space for a struct sx, and description is a pointer to a null-termi- nated character string that describes the shared/exclusive lock. Shared/exclusive locks are destroyed with sx_destroy(). Threads acquire and release a shared lock by calling sx_slock() or sx_try_slock() and sx_sunlock(). Threads acquire and release an exclusive lock by calling sx_xlock() or sx_try_xlock() and sx_xunlock(). A thread can attempt to upgrade a currently owned shared lock to an exclusive lock by calling sx_try_upgrade(). A thread that owns an exclusive lock can downgrade it to a shared lock by calling sx_downgrade(). sx_try_slock() and sx_try_xlock() will return 0 if the shared/exclusive lock cannot be acquired immediately; otherwise the shared/exclusive lock will be acquired and a non-zero value will be returned. sx_try_upgrade() will return 0 if the shared lock cannot be upgraded to an exclusive lock immediately; otherwise the exclusive lock will be ac- quired and a non-zero value will be returned. The sx_assert() function tests specified conditions and panics if they are not met and the kernel is compiled with INVARIANTS. The SX_SYSINIT() macro is used to generate a call to the sx_sysinit() routine at system startup in order to initialize a given sx lock. The parameters are the same as sx_init() but with an additional argument, name, that is used in generating unique variable names for the related structures associated with the lock and the sysinit routine. The following assertions are supported: SX_LOCKED Assert that the current thread has either a shared or an ex- clusive lock on the sx lock pointed to by the first argument. SX_SLOCKED Assert that the current thread has a shared lock on the sx lock pointed to by the first argument. SX_XLOCKED Assert that the current thread has an exclusive lock on the sx lock pointed to by the first argument. A thread may not own a shared lock and an exclusive lock simultaneously; attempting to do so will result in deadlock. CONTEXT It is allowed to own a shared lock or an exclusive lock while sleeping. SEE ALSO condvar(9), mtx_pool(9), mutex(9), sema(9) BUGS Currently there is no way to assert that a lock is not held. This is not possible in the non-WITNESS case for asserting that this thread does not hold a shared lock. In the non-WITNESS case, the SX_LOCKED and SX_SLOCKED assertions merely check that some thread holds a shared lock. They do not ensure that the current thread holds a shared lock. BSD August 14, 2001 BSD
NAME | SYNOPSIS | DESCRIPTION | CONTEXT | SEE ALSO | BUGS
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