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.Nd kernel shared/exclusive lock 46.Sh SYNOPSIS 47.In sys/param.h 48.In sys/lock.h 49.In sys/mutex.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.Sh DESCRIPTION 74Shared/exclusive locks are used to protect data that are read far more often 75than they are written. 76Mutexes are inherently more efficient than shared/exclusive locks, so 77shared/exclusive locks should be used prudently. 78.Pp 79Shared/exclusive locks are created with 80.Fn sx_init , 81where 82.Fa sx 83is a pointer to space for a 84.Vt struct sx , 85and 86.Fa description 87is a pointer to a null-terminated character string that describes the 88shared/exclusive lock. 89Shared/exclusive locks are destroyed with 90.Fn sx_destroy . 91Threads acquire and release a shared lock by calling 92.Fn sx_slock 93or 94.Fn sx_try_slock 95and 96.Fn sx_sunlock . 97Threads acquire and release an exclusive lock by calling 98.Fn sx_xlock 99or 100.Fn sx_try_xlock 101and 102.Fn sx_xunlock . 103A thread can attempt to upgrade a currently owned shared lock to an exclusive 104lock by calling 105.Fn sx_try_upgrade . 106A thread that owns an exclusive lock can downgrade it to a shared lock by 107calling 108.Fn sx_downgrade . 109.Pp 110.Fn sx_try_slock 111and 112.Fn sx_try_xlock 113will return 0 if the shared/exclusive lock cannot be acquired immediately; 114otherwise the shared/exclusive lock will be acquired and a non-zero value will 115be returned. 116.Pp 117.Fn sx_try_upgrade 118will return 0 if the shared lock cannot be upgraded to an exclusive lock 119immediately; otherwise the exclusive lock will be acquired and a non-zero value 120will be returned. 121.Pp 122The 123.Fn sx_assert 124function tests specified conditions and panics if they are not met and the 125kernel is compiled with 126.Dv INVARIANTS . 127The following assertions are supported: 128.Bl -tag -width ".Dv SX_XLOCKED" 129.It Dv SX_LOCKED 130Assert that the current thread has either a shared or an exclusive lock on the 131.Vt sx 132lock pointed to by the first argument. 133.It Dv SX_SLOCKED 134Assert that the current thread has a shared lock on the 135.Vt sx 136lock pointed to by 137the first argument. 138.It Dv SX_XLOCKED 139Assert that the current thread has an exclusive lock on the 140.Vt sx 141lock pointed to 142by the first argument. 143.El 144.Pp 145A thread may not own a shared lock and an exclusive lock simultaneously; 146attempting to do so will result in deadlock. 147.Sh SEE ALSO 148.Xr condvar 9 , 149.Xr mutex 9 , 150.Xr sema 9 151.Sh BUGS 152Currently there is no way to assert that a lock is not held. 153This is not possible in the 154.No non- Ns Dv WITNESS 155case for asserting that this thread 156does not hold a shared lock. 157In the 158.No non- Ns Dv WITNESS 159case, the 160.Dv SX_LOCKED 161and 162.Dv SX_SLOCKED 163assertions merely check that some thread holds a shared lock. 164They do not ensure that the current thread holds a shared lock. 165