/share/man/man4/witness.4

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 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 .