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SX(9) FreeBSD 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_unlock, SX_SYSINIT -- kernel shared/exclusive lock SYNOPSIS #include <sys/param.h> #include <sys/lock.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 utility macros sx_unlock(struct sx *sx); SX_SYSINIT(name, struct sx *sx, const char *description); Kernel options options INVARIANTS options INVARIANT_SUPPORT 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() or sx_unlock(). Threads acquire and release an exclusive lock by calling sx_xlock() or sx_try_xlock() and sx_xunlock() or sx_unlock(). A thread can attempt to upgrade a currently held shared lock to an exclusive lock by calling sx_try_upgrade(). A thread that has 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 acquired and a non-zero value will be returned. When compiled with options INVARIANTS and options INVARIANT_SUPPORT, the sx_assert() function tests sx for the assertions specified in what, and panics if they are not met. The following assertions are supported: SX_LOCKED Assert that the current thread has either a shared or an exclusive lock on the sx lock pointed to by the first argu- ment. 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. SX_UNLOCKED Assert that the current thread has no lock on the sx lock pointed to by the first argument. For ease of programming, sx_unlock() is provided as a macro frontend to the respective functions, sx_sunlock() and sx_xunlock(). Algorithms that are aware of what state the lock is in should use either of the two spe- cific functions for a minor performance benefit. 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. A thread may not hold both a shared lock and an exclusive lock on the same lock simultaneously; attempting to do so will result in deadlock. CONTEXT A thread may hold a shared or exclusive lock on an sx lock while sleep- ing. As a result, an sx lock may not be acquired while holding a mutex. Otherwise, if one thread slept while holding an sx lock while another thread blocked on the same sx lock after acquiring a mutex, then the sec- ond thread would effectively end up sleeping while holding a mutex, which is not allowed. SEE ALSO condvar(9), mtx_pool(9), mutex(9), panic(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. FreeBSD 11.1 August 14, 2001 FreeBSD 11.1
NAME | SYNOPSIS | DESCRIPTION | CONTEXT | SEE ALSO | BUGS
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