1.\" 2.\" Copyright (C) 2001 Jason Evans <jasone@FreeBSD.org>. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice(s), this list of conditions and the following disclaimer as 9.\" the first lines of this file unmodified other than the possible 10.\" addition of one or more copyright notices. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice(s), this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 15.\" THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER(S) ``AS IS'' AND ANY 16.\" EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED 17.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE 18.\" DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE LIABLE FOR ANY 19.\" DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES 20.\" (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR 21.\" SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER 22.\" CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 23.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 24.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH 25.\" DAMAGE. 26.\" 27.\" $FreeBSD$ 28.\" 29.Dd August 14, 2001 30.Dt SX 9 31.Os 32.Sh NAME 33.Nm sx , 34.Nm sx_init , 35.Nm sx_destroy , 36.Nm sx_slock , 37.Nm sx_xlock , 38.Nm sx_try_slock , 39.Nm sx_try_xlock , 40.Nm sx_sunlock , 41.Nm sx_xunlock , 42.Nm sx_try_upgrade , 43.Nm sx_downgrade , 44.Nm sx_assert , 45.Nm SX_SYSINIT 46.Nd kernel shared/exclusive lock 47.Sh SYNOPSIS 48.In sys/param.h 49.In sys/lock.h 50.In sys/sx.h 51.Ft void 52.Fn sx_init "struct sx *sx" "const char *description" 53.Ft void 54.Fn sx_destroy "struct sx *sx" 55.Ft void 56.Fn sx_slock "struct sx *sx" 57.Ft void 58.Fn sx_xlock "struct sx *sx" 59.Ft int 60.Fn sx_try_slock "struct sx *sx" 61.Ft int 62.Fn sx_try_xlock "struct sx *sx" 63.Ft void 64.Fn sx_sunlock "struct sx *sx" 65.Ft void 66.Fn sx_xunlock "struct sx *sx" 67.Ft int 68.Fn sx_try_upgrade "struct sx *sx" 69.Ft void 70.Fn sx_downgrade "struct sx *sx" 71.Ft void 72.Fn sx_assert "struct sx *sx" "int what" 73.In sys/kernel.h 74.Fn SX_SYSINIT "name" "struct sx *sx" "const char *description" 75.Sh DESCRIPTION 76Shared/exclusive locks are used to protect data that are read far more often 77than they are written. 78Mutexes are inherently more efficient than shared/exclusive locks, so 79shared/exclusive locks should be used prudently. 80.Pp 81Shared/exclusive locks are created with 82.Fn sx_init , 83where 84.Fa sx 85is a pointer to space for a 86.Vt struct sx , 87and 88.Fa description 89is a pointer to a null-terminated character string that describes the 90shared/exclusive lock. 91Shared/exclusive locks are destroyed with 92.Fn sx_destroy . 93Threads acquire and release a shared lock by calling 94.Fn sx_slock 95or 96.Fn sx_try_slock 97and 98.Fn sx_sunlock . 99Threads acquire and release an exclusive lock by calling 100.Fn sx_xlock 101or 102.Fn sx_try_xlock 103and 104.Fn sx_xunlock . 105A thread can attempt to upgrade a currently owned shared lock to an exclusive 106lock by calling 107.Fn sx_try_upgrade . 108A thread that owns an exclusive lock can downgrade it to a shared lock by 109calling 110.Fn sx_downgrade . 111.Pp 112.Fn sx_try_slock 113and 114.Fn sx_try_xlock 115will return 0 if the shared/exclusive lock cannot be acquired immediately; 116otherwise the shared/exclusive lock will be acquired and a non-zero value will 117be returned. 118.Pp 119.Fn sx_try_upgrade 120will return 0 if the shared lock cannot be upgraded to an exclusive lock 121immediately; otherwise the exclusive lock will be acquired and a non-zero value 122will be returned. 123.Pp 124The 125.Fn sx_assert 126function tests specified conditions and panics if they are not met and the 127kernel is compiled with 128.Dv INVARIANTS . 129The following assertions are supported: 130.Bl -tag -width ".Dv SX_XLOCKED" 131.It Dv SX_LOCKED 132Assert that the current thread has either a shared or an exclusive lock on the 133.Vt sx 134lock pointed to by the first argument. 135.It Dv SX_SLOCKED 136Assert that the current thread has a shared lock on the 137.Vt sx 138lock pointed to by 139the first argument. 140.It Dv SX_XLOCKED 141Assert that the current thread has an exclusive lock on the 142.Vt sx 143lock pointed to 144by the first argument. 145.El 146.Pp 147The 148.Fn SX_SYSINIT 149macro is used to generate a call to the 150.Fn sx_sysinit 151routine at system startup in order to initialize a given 152.Fa sx 153lock. 154The parameters are the same as 155.Fn sx_init 156but with an additional argument, 157.Fa name , 158that is used in generating unique variable names for the related 159structures associated with the lock and the sysinit routine. 160.Pp 161A thread may not own a shared lock and an exclusive lock simultaneously; 162attempting to do so will result in deadlock. 163.Sh CONTEXT 164It is allowed to own a shared lock or an exclusive lock while sleeping. 165.Sh SEE ALSO 166.Xr condvar 9 , 167.Xr mtx_pool 9 , 168.Xr mutex 9 , 169.Xr sema 9 170.Sh BUGS 171Currently there is no way to assert that a lock is not held. 172This is not possible in the 173.No non- Ns Dv WITNESS 174case for asserting that this thread 175does not hold a shared lock. 176In the 177.No non- Ns Dv WITNESS 178case, the 179.Dv SX_LOCKED 180and 181.Dv SX_SLOCKED 182assertions merely check that some thread holds a shared lock. 183They do not ensure that the current thread holds a shared lock. 184