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Man Pages


Manual Reference Pages  - SX (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_xlocked, SX_SYSINIT - kernel shared/exclusive lock

CONTENTS

Synopsis
     Nm Ss utility macros
     Kernel options
Description
Context
See Also
Bugs

SYNOPSIS


.In sys/param.h
.In sys/lock.h
.In 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 int sx_xlocked struct sx *sx

    Nm Ss utility macros

sx_unlock struct sx *sx SX_SYSINIT name struct sx *sx const char *description

    Kernel options


.Cd options INVARIANTS
.Cd options INVARIANT_SUPPORT

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
.Vt 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
.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. The following assertions are supported:
SX_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.
SX_SLOCKED Assert that the current thread has a shared lock on the
.Vt sx lock pointed to by the first argument.
SX_XLOCKED Assert that the current thread has an exclusive lock on the
.Vt sx lock pointed to by the first argument.
SX_UNLOCKED
  Assert that the current thread has no lock on the
.Vt sx lock pointed to by the first argument.

sx_xlocked will return non-zero if the current process 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

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.
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January 5, 2005 SX (9)

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