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/mutex.h 51.In sys/sx.h 52.Ft void 53.Fn sx_init "struct sx *sx" "const char *description" 54.Ft void 55.Fn sx_destroy "struct sx *sx" 56.Ft void 57.Fn sx_slock "struct sx *sx" 58.Ft void 59.Fn sx_xlock "struct sx *sx" 60.Ft int 61.Fn sx_try_slock "struct sx *sx" 62.Ft int 63.Fn sx_try_xlock "struct sx *sx" 64.Ft void 65.Fn sx_sunlock "struct sx *sx" 66.Ft void 67.Fn sx_xunlock "struct sx *sx" 68.Ft int 69.Fn sx_try_upgrade "struct sx *sx" 70.Ft void 71.Fn sx_downgrade "struct sx *sx" 72.Ft void 73.Fn sx_assert "struct sx *sx" "int what" 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 . 129.Pp 130The 131.Fn SX_SYSINIT 132macro is used to generate a call to the 133.Fn sx_sysinit 134routine at system startup in order to initialize a given 135.Fa sx 136lock. 137The parameters are the same as 138.Fn sx_init 139but with an additional argument, 140.Fa name , 141that is used in generating unique variable names for the related 142structures associated with the lock and the sysinit routine. 143.Pp 144The following assertions are supported: 145.Bl -tag -width ".Dv SX_XLOCKED" 146.It Dv SX_LOCKED 147Assert that the current thread has either a shared or an exclusive lock on the 148.Vt sx 149lock pointed to by the first argument. 150.It Dv SX_SLOCKED 151Assert that the current thread has a shared lock on the 152.Vt sx 153lock pointed to by 154the first argument. 155.It Dv SX_XLOCKED 156Assert that the current thread has an exclusive lock on the 157.Vt sx 158lock pointed to 159by the first argument. 160.El 161.Pp 162A thread may not own a shared lock and an exclusive lock simultaneously; 163attempting to do so will result in deadlock. 164.Sh CONTEXT 165It is allowed to own a shared lock or an exclusive lock while sleeping. 166.Sh SEE ALSO 167.Xr condvar 9 , 168.Xr mtx_pool 9 , 169.Xr mutex 9 , 170.Xr sema 9 171.Sh BUGS 172Currently there is no way to assert that a lock is not held. 173This is not possible in the 174.No non- Ns Dv WITNESS 175case for asserting that this thread 176does not hold a shared lock. 177In the 178.No non- Ns Dv WITNESS 179case, the 180.Dv SX_LOCKED 181and 182.Dv SX_SLOCKED 183assertions merely check that some thread holds a shared lock. 184They do not ensure that the current thread holds a shared lock. 185