Provided by: freebsd-manpages_6.2-1_all
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_xlocked, SX_SYSINIT - kernel shared/exclusive lock
sx_init(struct sx *sx, const char *description);
sx_destroy(struct sx *sx);
sx_slock(struct sx *sx);
sx_xlock(struct sx *sx);
sx_try_slock(struct sx *sx);
sx_try_xlock(struct sx *sx);
sx_sunlock(struct sx *sx);
sx_xunlock(struct sx *sx);
sx_try_upgrade(struct sx *sx);
sx_downgrade(struct sx *sx);
sx_assert(struct sx *sx, int what);
sx_xlocked(struct sx *sx);
sx utility macros
sx_unlock(struct sx *sx);
SX_SYSINIT(name, struct sx *sx, const char *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
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-
terminated 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_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
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.
sx_xlocked() will return non-zero if the current process holds the
exclusive lock; otherwise, it will return zero.
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
specific 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.
A thread may hold a shared or exclusive lock on an sx lock while
sleeping. 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 second thread would effectively end up sleeping while holding a
mutex, which is not allowed.
condvar(9), mtx_pool(9), mutex(9), panic(9), sema(9)
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.