--- name: "sx_init" section: "9freebsd" platform: "linux" package: "freebsd-manpages" version: "12.2-2" release: "Debian 13 (Trixie)" platforms: ["linux"] url: "https://new.carta.tech/SX_SYSINIT/9freebsd" --- # sx\_init(9freebsd) kernel shared/exclusive lock Part of [SX\_SYSINIT(9freebsd)](/SX_SYSINIT/9freebsd.md). ## 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**, **SX\_SYSINIT\_FLAGS** – kernel shared/exclusive lock ## SYNOPSIS **#include \** **#include \** **#include \** *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*); **options INVARIANTS** **options INVARIANT\_SUPPORT** *void*\ **sx\_assert**(*const struct sx \*sx*, *int what*); **#include \** **SX\_SYSINIT**(*name*, *struct sx \*sx*, *const char \*desc*); **SX\_SYSINIT\_FLAGS**(*name*, *struct sx \*sx*, *const char \*desc*, *int flags*); ## 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: - SX\_NOADAPTIVE 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**. - SX\_DUPOK Witness should not log messages about duplicate locks being acquired. - SX\_NOWITNESS Instruct [witness(4)](/witness/4.md) 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)](/ktr/4.md). - SX\_NEW 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 **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)](/sleep/9.md). 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. 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 *sx* lock pointed to by the first argument. - SA\_SLOCKED Assert that the current thread has a shared lock on the *sx* lock pointed to by the first argument. - SA\_XLOCKED Assert that the current thread has an exclusive lock on the *sx* lock pointed to by the first argument. - SA\_UNLOCKED 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: - SA\_RECURSED Assert that the current thread has a recursed lock on *sx*. - SA\_NOTRECURSED 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. The **SX\_SYSINIT\_FLAGS**() macro can similarly be used to initialize a given *sx* lock using **sx\_init\_flags**(). 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 [lock(9)](/lock/9.md), [locking(9)](/locking/9.md), [mutex(9)](/mutex/9.md), [panic(9)](/panic/9.md), [rwlock(9)](/rwlock/9.md), [sema(9)](/sema/9.md) ## BUGS 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 *any* 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.