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 November 18, 2015 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_COUNT 36.Cd options WITNESS_KDB 37.Cd options WITNESS_NO_VNODE 38.Cd options WITNESS_SKIPSPIN 39.Sh DESCRIPTION 40The 41.Nm 42module keeps track of the locks acquired and released by each thread. 43It also keeps track of the order in which locks are acquired with respect 44to each other. 45Each time a lock is acquired, 46.Nm 47uses these two lists to verify that a lock is not being acquired in the 48wrong order. 49If a lock order violation is detected, then a message is output to the 50kernel console or log detailing the locks involved and the locations in 51question. 52Witness can also be configured to drop into the kernel debugger when an order 53violation occurs. 54.Pp 55The 56.Nm 57code also checks various other conditions such as verifying that one 58does not recurse on a non-recursive lock, 59or attempt an upgrade on a shared lock held by another thread. 60If any of these checks fail, then the kernel will panic. 61.Pp 62The 63.Dv WITNESS_COUNT 64kernel option controls the maximum number of 65.Nm 66entries that are tracked in the kernel. 67The maximum number of entries can be queried via the 68.Va debug.witness.count 69sysctl. 70It can also be set from the 71.Xr loader 8 72via the 73.Va debug.witness.count 74environment variable. 75.Pp 76The 77.Dv WITNESS_NO_VNODE 78kernel option tells 79.Nm 80to ignore locking issues between 81.Xr vnode 9 82objects. 83.Pp 84The flag that controls whether or not the kernel debugger is entered when a 85lock order violation is detected can be set in a variety of ways. 86By default, the flag is off, but if the 87.Dv WITNESS_KDB 88kernel option is 89specified, then the flag will default to on. 90It can also be set from the 91.Xr loader 8 92via the 93.Va debug.witness.kdb 94environment variable or after the kernel has booted via the 95.Va debug.witness.kdb 96sysctl. 97If the flag is set to zero, then the debugger will not be entered. 98If the flag is non-zero, then the debugger will be entered. 99.Pp 100The 101.Nm 102code can also be configured to skip all checks on spin mutexes. 103By default, this flag defaults to off, but it can be turned on by 104specifying the 105.Dv WITNESS_SKIPSPIN 106kernel option. 107The flag can also be set via the 108.Xr loader 8 109environment variable 110.Va debug.witness.skipspin . 111If the variable is set to a non-zero value, then spin mutexes are skipped. 112Once the kernel has booted, the status of this flag can be examined but not 113set via the read-only sysctl 114.Va debug.witness.skipspin . 115.Pp 116The sysctl 117.Va debug.witness.watch 118specifies the level of witness involvement in the system. 119A value of 1 specifies that witness is enabled. 120A value of 0 specifies that witness is disabled, but that can be enabled 121again. 122This will maintain a small amount of overhead in the system. 123A value of -1 specifies that witness is disabled permanently and 124cannot be enabled again. 125The sysctl 126.Va debug.witness.watch 127can be set via 128.Xr loader 8 . 129.Pp 130The sysctl 131.Va debug.witness.output_channel 132specifies the output channel used to display warnings emitted by 133.Nm . 134The possible values are 135.Ql console , 136indicating that warnings are to be printed to the system console, 137.Ql log , 138indicating that warnings are to be logged via 139.Xr log 9 , 140and 141.Ql none . 142This sysctl can be set via 143.Xr loader 8 . 144.Pp 145The 146.Nm 147code also provides three extra 148.Xr ddb 4 149commands if both 150.Nm 151and 152.Xr ddb 4 153are compiled into the kernel: 154.Bl -ohang 155.It Ic show locks Op thread 156Outputs the list of locks held by a thread to the kernel console 157along with the filename and line number at which each lock was last acquired 158by the thread. 159The optional 160.Ar thread 161argument may be either a TID, 162PID, 163or pointer to a thread structure. 164If 165.Ar thread 166is not specified, 167then the locks held by the current thread are displayed. 168.It Ic show all locks 169Outputs the list of locks held by all threads in the system to the 170kernel console. 171.It Ic show witness 172Dump the current order list to the kernel console. 173The code first displays the lock order tree for all of the sleep locks. 174Then it displays the lock order tree for all of the spin locks. 175Finally, it displays a list of locks that have not yet been acquired. 176.El 177.Sh SEE ALSO 178.Xr ddb 4 , 179.Xr loader 8 , 180.Xr sysctl 8 , 181.Xr mutex 9 182.Sh HISTORY 183The 184.Nm 185code first appeared in 186.Bsx 5.0 187and was imported from there into 188.Fx 5.0 . 189