1.\" Copyright (c) 2001 John H. Baldwin <jhb@FreeBSD.org> 2.\" 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, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 13.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 14.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 15.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 16.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 17.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 18.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 19.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 20.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 21.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 22.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 23.\" SUCH DAMAGE. 24.\" 25.\" $FreeBSD$ 26.\" 27.Dd February 18, 2001 28.Dt WITNESS 4 29.Os 30.Sh NAME 31.Nm witness 32.Nd lock validation facility 33.Sh SYNOPSIS 34.Cd options WITNESS 35.Cd options WITNESS_KDB 36.Cd options WITNESS_SKIPSPIN 37.Sh DESCRIPTION 38The 39.Nm 40module keeps track of the locks acquired and released by each thread. 41It also keeps track of the order in which locks are acquired with respect 42to each other. 43Each time a lock is acquired, 44.Nm 45uses these two lists to verify that a lock is not being acquired in the 46wrong order. 47If a lock order violation is detected, then a message is output to the 48kernel console detailing the locks involved and the locations in question. 49Witness can also be configured to drop into the kernel debugger when an order 50violation occurs. 51.Pp 52The 53.Nm 54code also checks various other conditions such as verifying that one 55does not recurse on a non-recursive lock. 56For sleep locks, 57.Nm 58verifies that a new process would not be switched to when a lock is released 59or a lock is blocked on during an acquire while any spin locks are held. 60If any of these checks fail, then the kernel will panic. 61.Pp 62The flag that controls whether or not the kernel debugger is entered when a 63lock order violation is detected can be set in a variety of ways. 64By default, the flag is off, but if the 65.Dv WITNESS_KDB 66kernel option is 67specified, then the flag will default to on. 68It can also be set from the 69.Xr loader 8 70via the 71.Va debug.witness.kdb 72environment variable or after the kernel has booted via the 73.Va debug.witness.kdb 74sysctl. 75If the flag is set to zero, then the debugger will not be entered. 76If the flag is non-zero, then the debugger will be entered. 77.Pp 78The 79.Nm 80code can also be configured to skip all checks on spin mutexes. 81By default, this flag defaults to off, but it can be turned on by 82specifying the 83.Dv WITNESS_SKIPSPIN 84kernel option. 85The flag can also be set via the 86.Xr loader 8 87environment variable 88.Va debug.witness.skipspin . 89If the variable is set to a non-zero value, then spin mutexes are skipped. 90Once the kernel has booted, the status of this flag can be examined but not 91set via the read-only sysctl 92.Va debug.witness.skipspin . 93.Pp 94The sysctl 95.Va debug.witness.watch 96specifies the level of witness involvement in the system. 97A value of 1 specifies that witness is enabled. 98A value of 0 specifies that witness is disabled, but that can be enabled 99again. This will maintain a small amount of overhead in the system. 100A value of -1 specifies that witness is disabled permanently and that 101cannot be enabled again. 102The sysctl 103.Va debug.witness.watch 104can be set via 105.Xr loader 8 . 106.Pp 107The 108.Nm 109code also provides two extra 110.Xr ddb 4 111commands if both 112.Nm 113and 114.Xr ddb 4 115are compiled into the kernel: 116.Bl -ohang 117.It Ic show locks 118Outputs the list of locks held by the current thread to the kernel console 119along with the filename and line number at which each lock was last acquired 120by this thread. 121.It Ic show witness 122Dump the current order list to the kernel console. 123The code first displays the lock order tree for all of the sleep locks. 124Then it displays the lock order tree for all of the spin locks. 125Finally, it displays a list of locks that have not yet been acquired. 126.El 127.Sh SEE ALSO 128.Xr ddb 4 , 129.Xr loader 8 , 130.Xr sysctl 8 , 131.Xr mutex 9 132.Sh HISTORY 133The 134.Nm 135code first appeared in 136.Bsx 5.0 137and was imported from there into 138.Fx 5.0 . 139.Sh BUGS 140The 141.Nm 142code currently does not handle recursion of shared 143.Xr sx 9 144locks properly. 145