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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 May 30, 2012 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, 56or attempt an upgrade on a shared lock held by another thread. 57If any of these checks fail, then the kernel will panic. 58.Pp 59The flag that controls whether or not the kernel debugger is entered when a 60lock order violation is detected can be set in a variety of ways. 61By default, the flag is off, but if the 62.Dv WITNESS_KDB 63kernel option is 64specified, then the flag will default to on. 65It can also be set from the 66.Xr loader 8 67via the 68.Va debug.witness.kdb 69environment variable or after the kernel has booted via the 70.Va debug.witness.kdb 71sysctl. 72If the flag is set to zero, then the debugger will not be entered. 73If the flag is non-zero, then the debugger will be entered. 74.Pp 75The 76.Nm 77code can also be configured to skip all checks on spin mutexes. 78By default, this flag defaults to off, but it can be turned on by 79specifying the 80.Dv WITNESS_SKIPSPIN 81kernel option. 82The flag can also be set via the 83.Xr loader 8 84environment variable 85.Va debug.witness.skipspin . 86If the variable is set to a non-zero value, then spin mutexes are skipped. 87Once the kernel has booted, the status of this flag can be examined but not 88set via the read-only sysctl 89.Va debug.witness.skipspin . 90.Pp 91The sysctl 92.Va debug.witness.watch 93specifies the level of witness involvement in the system. 94A value of 1 specifies that witness is enabled. 95A value of 0 specifies that witness is disabled, but that can be enabled 96again. This will maintain a small amount of overhead in the system. 97A value of -1 specifies that witness is disabled permanently and 98cannot be enabled again. 99The sysctl 100.Va debug.witness.watch 101can be set via 102.Xr loader 8 . 103.Pp 104The 105.Nm 106code also provides two extra 107.Xr ddb 4 108commands if both 109.Nm 110and 111.Xr ddb 4 112are compiled into the kernel: 113.Bl -ohang 114.It Ic show locks Op thread 115Outputs the list of locks held by a thread to the kernel console 116along with the filename and line number at which each lock was last acquired 117by the thread. 118The optional 119.Ar thread 120argument may be either a TID, 121PID, 122or pointer to a thread structure. 123If 124.Ar thread 125is not specified, 126then the locks held by the current thread are displayed. 127.It Ic show all locks 128Outputs the list of locks held by all threads in the system to the 129kernel console. 130.It Ic show witness 131Dump the current order list to the kernel console. 132The code first displays the lock order tree for all of the sleep locks. 133Then it displays the lock order tree for all of the spin locks. 134Finally, it displays a list of locks that have not yet been acquired. 135.El 136.Sh SEE ALSO 137.Xr ddb 4 , 138.Xr loader 8 , 139.Xr sysctl 8 , 140.Xr mutex 9 141.Sh HISTORY 142The 143.Nm 144code first appeared in 145.Bsx 5.0 146and was imported from there into 147.Fx 5.0 .