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_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);
sx utility macros
sx_unlock(struct sx *sx);
SX_SYSINIT(name, struct sx *sx, const char *description);
Kernel options
options INVARIANTS
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 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 call

ing
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

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 ex

clusive lock on

the sx lock pointed to by the first argu

ment.

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.

BSD January 5, 2005

BSD