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31 .Nd lock validation facility
34 .Cd options WITNESS_COUNT
35 .Cd options WITNESS_KDB
36 .Cd options WITNESS_NO_VNODE
37 .Cd options WITNESS_SKIPSPIN
41 module keeps track of the locks acquired and released by each thread.
42 It also keeps track of the order in which locks are acquired with respect
44 Each time a lock is acquired,
46 uses these two lists to verify that a lock is not being acquired in the
48 If a lock order violation is detected, then a message is output to the
49 kernel console or log detailing the locks involved and the locations in
51 Witness can also be configured to drop into the kernel debugger when an order
56 code also checks various other conditions such as verifying that one
57 does not recurse on a non-recursive lock,
58 or attempt an upgrade on a shared lock held by another thread.
59 If any of these checks fail, then the kernel will panic.
63 kernel option controls the maximum number of
65 entries that are tracked in the kernel.
66 The maximum number of entries can be queried via the
67 .Va debug.witness.count
69 It can also be set from the
72 .Va debug.witness.count
79 to ignore locking issues between
83 The flag that controls whether or not the kernel debugger is entered when a
84 lock order violation is detected can be set in a variety of ways.
85 By default, the flag is off, but if the
88 specified, then the flag will default to on.
89 It can also be set from the
93 environment variable or after the kernel has booted via the
96 If the flag is set to zero, then the debugger will not be entered.
97 If the flag is non-zero, then the debugger will be entered.
101 code can also be configured to skip all checks on spin mutexes.
102 By default, this flag defaults to off, but it can be turned on by
106 The flag can also be set via the
109 .Va debug.witness.skipspin .
110 If the variable is set to a non-zero value, then spin mutexes are skipped.
111 Once the kernel has booted, the status of this flag can be examined but not
112 set via the read-only sysctl
113 .Va debug.witness.skipspin .
116 .Va debug.witness.watch
117 specifies the level of witness involvement in the system.
118 A value of 1 specifies that witness is enabled.
119 A value of 0 specifies that witness is disabled, but that can be enabled
121 This will maintain a small amount of overhead in the system.
122 A value of -1 specifies that witness is disabled permanently and
123 cannot be enabled again.
125 .Va debug.witness.watch
130 .Va debug.witness.output_channel
131 specifies the output channel used to display warnings emitted by
133 The possible values are
135 indicating that warnings are to be printed to the system console,
137 indicating that warnings are to be logged via
141 This sysctl can be set via
146 code also provides three extra
152 are compiled into the kernel:
154 .It Ic show locks Op thread
155 Outputs the list of locks held by a thread to the kernel console
156 along with the filename and line number at which each lock was last acquired
160 argument may be either a TID,
162 or pointer to a thread structure.
166 then the locks held by the current thread are displayed.
167 .It Ic show all locks
168 Outputs the list of locks held by all threads in the system to the
171 Dump the current order list to the kernel console.
172 The code first displays the lock order tree for all of the sleep locks.
173 Then it displays the lock order tree for all of the spin locks.
174 Finally, it displays a list of locks that have not yet been acquired.
184 code first appeared in
186 and was imported from there into