freebsd-skq/share/man/man9/sx.9

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.Dd August 14, 2001
.Dt SX 9
.Os
.Sh NAME
.Nm sx ,
.Nm sx_init ,
.Nm sx_destroy ,
.Nm sx_slock ,
.Nm sx_xlock ,
.Nm sx_try_slock ,
.Nm sx_try_xlock ,
.Nm sx_sunlock ,
.Nm sx_xunlock ,
.Nm sx_try_upgrade ,
.Nm sx_downgrade ,
.Nm sx_assert,
.Nm SX_SYSINIT
.Nd kernel shared/exclusive lock
.Sh SYNOPSIS
.In sys/param.h
.In sys/lock.h
.In sys/mutex.h
.In sys/sx.h
.Ft void
.Fn sx_init "struct sx *sx" "const char *description"
.Ft void
.Fn sx_destroy "struct sx *sx"
.Ft void
.Fn sx_slock "struct sx *sx"
.Ft void
.Fn sx_xlock "struct sx *sx"
.Ft int
.Fn sx_try_slock "struct sx *sx"
.Ft int
.Fn sx_try_xlock "struct sx *sx"
.Ft void
.Fn sx_sunlock "struct sx *sx"
.Ft void
.Fn sx_xunlock "struct sx *sx"
.Ft int
.Fn sx_try_upgrade "struct sx *sx"
.Ft void
.Fn sx_downgrade "struct sx *sx"
.Ft void
.Fn sx_assert "struct sx *sx" "int what"
.Fn SX_SYSINIT "name" "struct sx *sx" "const char *description"
.Sh 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.
.Pp
Shared/exclusive locks are created with
.Fn sx_init ,
where
.Fa sx
is a pointer to space for a
.Vt struct sx ,
and
.Fa description
is a pointer to a null-terminated character string that describes the
shared/exclusive lock.
Shared/exclusive locks are destroyed with
.Fn sx_destroy .
Threads acquire and release a shared lock by calling
.Fn sx_slock
or
.Fn sx_try_slock
and
.Fn sx_sunlock .
Threads acquire and release an exclusive lock by calling
.Fn sx_xlock
or
.Fn sx_try_xlock
and
.Fn sx_xunlock .
A thread can attempt to upgrade a currently owned shared lock to an exclusive
lock by calling
.Fn sx_try_upgrade .
A thread that owns an exclusive lock can downgrade it to a shared lock by
calling
.Fn sx_downgrade .
.Pp
.Fn sx_try_slock
and
.Fn 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.
.Pp
.Fn 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.
.Pp
The
.Fn sx_assert
function tests specified conditions and panics if they are not met and the
kernel is compiled with
.Dv INVARIANTS .
.Pp
The
.Fn SX_SYSINIT
macro is used to generate a call to the
.Fn sx_sysinit
routine at system startup in order to initialize a given sx lock.  The parameters are the same as
.Fn sx_init
but with an additional argument,
.Fa name,
that is used in generating unique variable names for the related structures associated with the lock and the sysinit routine.
.Pp
The following assertions are supported:
.Bl -tag -width ".Dv SX_XLOCKED"
.It Dv 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.
.It Dv SX_SLOCKED
Assert that the current thread has a shared lock on the
.Vt sx
lock pointed to by
the first argument.
.It Dv SX_XLOCKED
Assert that the current thread has an exclusive lock on the
.Vt sx
lock pointed to
by the first argument.
.El
.Pp
A thread may not own a shared lock and an exclusive lock simultaneously;
attempting to do so will result in deadlock.
.Sh SEE ALSO
.Xr condvar 9 ,
.Xr mtx_pool 9 ,
.Xr mutex 9 ,
.Xr sema 9
.Sh BUGS
Currently there is no way to assert that a lock is not held.
This is not possible in the
.No non- Ns Dv WITNESS
case for asserting that this thread
does not hold a shared lock.
In the
.No non- Ns Dv WITNESS
case, the
.Dv SX_LOCKED
and
.Dv SX_SLOCKED
assertions merely check that some thread holds a shared lock.
They do not ensure that the current thread holds a shared lock.