.\" .\" Copyright (C) 2001 Jason Evans . All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice(s), this list of conditions and the following disclaimer as .\" the first lines of this file unmodified other than the possible .\" addition of one or more copyright notices. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice(s), this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" .\" THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER(S) ``AS IS'' AND ANY .\" EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED .\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE .\" DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE LIABLE FOR ANY .\" DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES .\" (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR .\" SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER .\" CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH .\" DAMAGE. .\" .\" $FreeBSD$ .\" .Dd February 1, 2006 .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_xlocked , .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" .Ft int .Fn sx_xlocked "struct sx *sx" .\" .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 .Fn sx_xlocked will return non-zero if the current process holds the exclusive lock; otherwise, it will return zero. .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 mutex 9 , .Xr panic 9 , .Xr rwlock 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.