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

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.Dd January 5, 2005
.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_unlock ,
.Nm SX_SYSINIT
.Nd kernel shared/exclusive lock
.Sh SYNOPSIS
.In sys/param.h
.In sys/lock.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"
.\"
.Ss Nm Ss utility macros
.Fn sx_unlock "struct sx *sx"
.Fn SX_SYSINIT "name" "struct sx *sx" "const char *description"
.\"
.Ss Kernel options
.Cd "options INVARIANTS"
.Cd "options INVARIANT_SUPPORT"
.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
or
.Fn sx_unlock .
Threads acquire and release an exclusive lock by calling
.Fn sx_xlock
or
.Fn sx_try_xlock
and
.Fn sx_xunlock
or
.Fn sx_unlock .
A thread can attempt to upgrade a currently held shared lock to an exclusive
lock by calling
.Fn sx_try_upgrade .
A thread that has 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
When compiled with
.Cd "options INVARIANTS"
and
.Cd "options INVARIANT_SUPPORT" ,
the
.Fn sx_assert
function tests
.Fa sx
for the assertions specified in
.Fa what ,
and panics if they are not met.
The following assertions are supported:
.Bl -tag -width ".Dv SX_UNLOCKED"
.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.
.It Dv SX_UNLOCKED
Assert that the current thread has no lock on the
.Vt sx
lock pointed to
by the first argument.
.El
.Pp
For ease of programming,
.Fn sx_unlock
is provided as a macro frontend to the respective functions,
.Fn sx_sunlock
and
.Fn 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.
.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
.Fa 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
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.
.Sh CONTEXT
A thread may hold a shared or exclusive lock on an
.Nm
lock while sleeping.
As a result, an
.Nm
lock may not be acquired while holding a mutex.
Otherwise, if one thread slept while holding an
.Nm
lock while another thread blocked on the same
.Nm
lock after acquiring a mutex, then the second thread would effectively
end up sleeping while holding a mutex, which is not allowed.
.Sh SEE ALSO
.Xr condvar 9 ,
.Xr mtx_pool 9 ,
.Xr mutex 9 ,
.Xr panic 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.