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32 .Nd lock validation facility
35 .Cd options WITNESS_COUNT
36 .Cd options WITNESS_KDB
37 .Cd options WITNESS_NO_VNODE
38 .Cd options WITNESS_SKIPSPIN
42 module keeps track of the locks acquired and released by each thread.
43 It also keeps track of the order in which locks are acquired with respect
45 Each time a lock is acquired,
47 uses these two lists to verify that a lock is not being acquired in the
49 If a lock order violation is detected, then a message is output to the
50 kernel console or log detailing the locks involved and the locations in
52 Witness can also be configured to drop into the kernel debugger when an order
57 code also checks various other conditions such as verifying that one
58 does not recurse on a non-recursive lock,
59 or attempt an upgrade on a shared lock held by another thread.
60 If any of these checks fail, then the kernel will panic.
64 kernel option controls the maximum number of
66 entries that are tracked in the kernel.
67 The maximum number of entries can be queried via the
68 .Va debug.witness.count
70 It can also be set from the
73 .Va debug.witness.count
80 to ignore locking issues between
84 The flag that controls whether or not the kernel debugger is entered when a
85 lock order violation is detected can be set in a variety of ways.
86 By default, the flag is off, but if the
89 specified, then the flag will default to on.
90 It can also be set from the
94 environment variable or after the kernel has booted via the
97 If the flag is set to zero, then the debugger will not be entered.
98 If the flag is non-zero, then the debugger will be entered.
102 code can also be configured to skip all checks on spin mutexes.
103 By default, this flag defaults to off, but it can be turned on by
107 The flag can also be set via the
110 .Va debug.witness.skipspin .
111 If the variable is set to a non-zero value, then spin mutexes are skipped.
112 Once the kernel has booted, the status of this flag can be examined but not
113 set via the read-only sysctl
114 .Va debug.witness.skipspin .
117 .Va debug.witness.watch
118 specifies the level of witness involvement in the system.
119 A value of 1 specifies that witness is enabled.
120 A value of 0 specifies that witness is disabled, but that can be enabled
122 This will maintain a small amount of overhead in the system.
123 A value of -1 specifies that witness is disabled permanently and
124 cannot be enabled again.
126 .Va debug.witness.watch
131 .Va debug.witness.output_channel
132 specifies the output channel used to display warnings emitted by
134 The possible values are
136 indicating that warnings are to be printed to the system console,
138 indicating that warnings are to be logged via
142 This sysctl can be set via
147 code also provides three extra
153 are compiled into the kernel:
155 .It Ic show locks Op thread
156 Outputs the list of locks held by a thread to the kernel console
157 along with the filename and line number at which each lock was last acquired
161 argument may be either a TID,
163 or pointer to a thread structure.
167 then the locks held by the current thread are displayed.
168 .It Ic show all locks
169 Outputs the list of locks held by all threads in the system to the
172 Dump the current order list to the kernel console.
173 The code first displays the lock order tree for all of the sleep locks.
174 Then it displays the lock order tree for all of the spin locks.
175 Finally, it displays a list of locks that have not yet been acquired.
185 code first appeared in
187 and was imported from there into