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SX(9) Kernel Developer's Manual SX(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_SYSINITkernel shared/exclusive lock


#include <sys/param.h>
#include <sys/lock.h>
#include <sys/sx.h>

sx_init(struct sx *sx, const char *description);

sx_init_flags(struct sx *sx, const char *description, int opts);

sx_destroy(struct sx *sx);

sx_slock(struct sx *sx);

sx_xlock(struct sx *sx);

sx_slock_sig(struct sx *sx);

sx_xlock_sig(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_unlock(struct sx *sx);

sx_try_upgrade(struct sx *sx);

sx_downgrade(struct sx *sx);

sx_sleep(void *chan, struct sx *sx, int priority, const char *wmesg, int timo);

struct thread *
sx_xholder(struct sx *sx);

sx_xlocked(const struct sx *sx);

sx_assert(const struct sx *sx, int what);

#include <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 () or () 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:

Disable adaptive spinning, rather than sleeping, for lock operations while an exclusive lock holder is executing on another CPU. Adaptive spinning is the default unless the kernel is compiled with options NO_ADAPTIVE_SX.
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).
If the kernel has been compiled with options INVARIANTS, sx_init() will assert that the sx has not been initialized multiple times without intervening calls to sx_destroy() unless this option is specified.

Shared/exclusive locks are destroyed with (). 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_sig() or sx_try_slock() and sx_sunlock() or sx_unlock(). Threads acquire and release an exclusive lock by calling (), 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 ().

() and () 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.

() 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.

() and () 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 (). For more details on the parameters to this function, see sleep(9).

When compiled with options INVARIANTS and options INVARIANT_SUPPORT, the () 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.

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

Assert that the current thread has a recursed lock on sx.
Assert that the current thread does not have a recursed lock on sx.

() 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.

() will return non-zero if the current thread holds the exclusive lock; otherwise, it will return zero.

For ease of programming, () is provided as a macro frontend to the respective functions, () and (). 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 () macro is used to generate a call to the () 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.


lock(9), locking(9), mutex(9), panic(9), rwlock(9), sema(9)


A kernel without WITNESS cannot assert whether the current thread does or does not hold a shared lock. SA_LOCKED and SA_SLOCKED can only assert that thread holds a shared lock. They cannot ensure that the current thread holds a shared lock. Further, SA_UNLOCKED can only assert that the current thread does not hold an exclusive lock.

March 28, 2016 Debian