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

SYNOPSIS
#include <sys/param.h>
#include <sys/lock.h>
#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);

options INVARIANTS
options INVARIANT_SUPPORT
void
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:

SX_NOADAPTIVE
    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.

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 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.

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-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.