|SX(9)||Kernel Developer's Manual||SX(9)|
NAME¶sx, sx_init, sx_init_flags, sx_destroy, sx_slock, sx_xlock, sx_slock_sig, sx_xlock_sig, sx_try_slock, sx_try_xlock, sx_sunlock, sx_xunlock, sx_unlock, sx_try_upgrade, sx_downgrade, sx_sleep, sx_xholder, sx_xlocked, sx_assert, SX_SYSINIT — kernel shared/exclusive lock
#include <sys/sx.h> void
sx_init(struct sx *sx, const char *description); void
sx_init_flags(struct sx *sx, const char *description, int opts); void
sx_destroy(struct sx *sx); void
sx_slock(struct sx *sx); void
sx_xlock(struct sx *sx); int
sx_slock_sig(struct sx *sx); int
sx_xlock_sig(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); void
sx_unlock(struct sx *sx); int
sx_try_upgrade(struct sx *sx); void
sx_downgrade(struct sx *sx); int
sx_sleep(void *chan, struct sx *sx, int priority, const char *wmesg, int timo); struct thread *
sx_xholder(struct sx *sx); int
sx_xlocked(struct sx *sx);
sx_assert(struct sx *sx, int what); #include <sys/kernel.h> 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. Shared/exclusive locks do not implement priority propagation like mutexes and reader/writer locks to prevent priority inversions, so shared/exclusive locks should be used prudently. Shared/exclusive locks are created with either sx_init() or sx_init_flags() 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. The opts argument to sx_init_flags() specifies a set of optional flags to alter the behavior of sx. It contains one or more of the following flags:
- If the kernel is not compiled with options NO_ADAPTIVE_SX, then lock operations for sx will spin instead of sleeping while an exclusive lock holder is executing on another CPU.
- Witness should not log messages about duplicate locks being acquired.
- Instruct witness(4) to ignore this lock.
- Do not profile this lock.
- Allow threads to recursively acquire exclusive locks for sx.
- Do not log any operations for this lock via ktr(4).
- Assert that the current thread has either a shared or an exclusive lock on the sx lock pointed to by the first argument.
- Assert that the current thread has a shared lock on the sx lock pointed to by the first argument.
- Assert that the current thread has an exclusive lock on the sx lock pointed to by the first argument.
- Assert that the current thread has no lock on the sx lock pointed to by the first argument.
- Assert that the current thread has a recursed lock on sx.
- Assert that the current thread does not have a recursed lock on sx.
NULLis returned instead. sx_xlocked() will return non-zero if the current thread 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.
CONTEXT¶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.
SEE ALSO¶locking(9), lock(9), mutex(9), panic(9), rwlock(9), sema(9)
BUGS¶Currently there is no way to assert that a lock is not held. This is not possible in the non-
WITNESScase for asserting that this thread does not hold a shared lock. In the non-
SA_SLOCKEDassertions merely check that some thread holds a shared lock. They do not ensure that the current thread holds a shared lock.
|May 28, 2009||Debian|