sx_try_upgrade(9)

NAME

sx, sx_init, sx_destroy, sx_slock, sx_xlock, sx_try_slock, sx_try_xlock, sx_sunlock, sx_xunlock, sx_try_upgrade, sx_downgrade, sx_assert, sx_unlock, 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_destroy(struct sx *sx);

void

sx_slock(struct sx *sx);

void

sx_xlock(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);

int

sx_try_upgrade(struct sx *sx);

void

sx_downgrade(struct sx *sx);

void

sx_assert(struct sx *sx, int what);

DESCRIPTION

Shared/exclusive locks are used to protect data that are read far more often than they are written. Mutexes are inherently more efficient than shared/exclusive locks, so shared/exclusive locks should be used prudently.

Shared/exclusive locks are created with sx_init(), 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. Shared/exclusive locks are destroyed with sx_destroy(). Threads acquire and release a shared lock by calling sx_slock() or sx_try_slock() and sx_sunlock() or sx_unlock(). Threads acquire and release an exclusive lock by calling sx_xlock() 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.

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. The following assertions are supported:

SX_LOCKED

Assert that the current thread has either a shared or an exclusive lock on the sx lock pointed to by the first argument.

SX_SLOCKED

Assert that the current thread has a shared lock on the sx lock pointed to by the first argument.

SX_XLOCKED

Assert that the current thread has an exclusive lock on the sx lock pointed to by the first argument.

SX_UNLOCKED

Assert that the current thread has no lock on the sx lock pointed to by the first argument.

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

condvar(9), mtx_pool(9), mutex(9), panic(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 SX_LOCKED and SX_SLOCKED assertions merely check that some thread holds a shared lock. They do not ensure that the current thread holds a shared lock.