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Man Pages

Manual Reference Pages  -  SX_XLOCK (9)


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


See Also


.In sys/param.h
.In sys/lock.h
.In 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 const struct sx *sx

.Cd options INVARIANTS
.Cd options INVARIANT_SUPPORT void sx_assert const struct sx *sx int what
.In sys/kernel.h 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. 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
.Vt 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
.Cd options NO_ADAPTIVE_SX , then lock operations for sx will spin instead of sleeping while an exclusive lock holder is executing on another CPU.
SX_DUPOK Witness should not log messages about duplicate locks being acquired.
SX_NOWITNESS Instruct witness(4) to ignore this lock.
SX_NOPROFILE Do not profile this lock.
SX_RECURSE Allow threads to recursively acquire exclusive locks for sx.
SX_QUIET Do not log any operations for this lock via ktr(4).

Shared/exclusive locks are destroyed with 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_slock, sx_slock_sig or sx_try_slock and sx_sunlock or sx_unlock. Threads acquire and release an exclusive lock by calling sx_xlock, sx_xlock_sig 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.

sx_slock_sig and 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
.Cd options INVARIANTS and
.Cd 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:
SA_LOCKED Assert that the current thread has either a shared or an exclusive lock on the
.Vt sx lock pointed to by the first argument.
SA_SLOCKED Assert that the current thread has a shared lock on the
.Vt sx lock pointed to by the first argument.
SA_XLOCKED Assert that the current thread has an exclusive lock on the
.Vt sx lock pointed to by the first argument.
  Assert that the current thread has no lock on the
.Vt sx lock pointed to by the first argument.

In addition, one of the following optional assertions may be included with either an SA_LOCKED, SA_SLOCKED, or SA_XLOCKED assertion:
SA_RECURSED 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 NULL is 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.


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.


locking(9), lock(9), mutex(9), panic(9), rwlock(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 SA_LOCKED and SA_SLOCKED assertions merely check that some thread holds a shared lock. They do not ensure that the current thread holds a shared lock.
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