|SX(9)||Kernel Developer's Manual||SX(9)|
kernel shared/exclusive lock
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(const struct sx *sx);
sx_assert(const 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. 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_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).
sx_destroy(). The lock sx must not be locked by any thread when it is destroyed. Threads acquire and release a shared lock by calling
sx_unlock(). Threads acquire and release an exclusive lock by calling
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_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.
sx_xlock_sig() do the same as their normal versions but performing an interruptible sleep. They return a non-zero value if the sleep has been interrupted by a signal or an interrupt, otherwise 0. A thread can atomically release a shared/exclusive lock while waiting for an event by calling
sx_sleep(). For more details on the parameters to this function, see sleep(9). When compiled with
options INVARIANT_SUPPORT, the
sx_assert() function tests sx for the assertions specified in what, and panics if they are not met. One of the following assertions must be specified:
- 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.
sx_xholder() will return a pointer to the thread which currently holds an exclusive lock on sx. If no thread holds an exclusive lock on sx, then
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_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
sxlock while sleeping. As a result, an
sxlock may not be acquired while holding a mutex. Otherwise, if one thread slept while holding an
sxlock while another thread blocked on the same
sxlock 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.
|November 16, 2011||Debian|