NAME
witness —
lock validation facility
SYNOPSIS
options WITNESS
options WITNESS_COUNT
options WITNESS_KDB
options WITNESS_NO_VNODE
options WITNESS_SKIPSPIN
DESCRIPTION
Thewitness module keeps track of the locks acquired and
released by each thread. It also keeps track of the order in which locks are
acquired with respect to each other. Each time a lock is acquired,
witness uses these two lists to verify that a lock is
not being acquired in the wrong order. If a lock order violation is detected,
then a message is output to the kernel console or log detailing the locks
involved and the locations in question. Witness can also be configured to drop
into the kernel debugger when an order violation occurs.
The witness code also checks various other
conditions such as verifying that one does not recurse on a non-recursive
lock, or attempt an upgrade on a shared lock held by another thread. If any
of these checks fail, then the kernel will panic.
The WITNESS_COUNT kernel option controls
the maximum number of witness entries that are
tracked in the kernel. The maximum number of entries can be queried via the
debug.witness.count sysctl. It can also be set from
the loader(8) via the debug.witness.count
environment variable.
The WITNESS_NO_VNODE kernel option tells
witness to ignore locking issues between
vnode(9) objects.
The flag that controls whether or not the kernel debugger is
entered when a lock order violation is detected can be set in a variety of
ways. By default, the flag is off, but if the
WITNESS_KDB kernel option is specified, then the
flag will default to on. It can also be set from the
loader(8) via the debug.witness.kdb environment
variable or after the kernel has booted via the
debug.witness.kdb sysctl. If the flag is set to zero,
then the debugger will not be entered. If the flag is non-zero, then the
debugger will be entered.
The witness code can also be configured to
skip all checks on spin mutexes. By default, this flag defaults to off, but
it can be turned on by specifying the
WITNESS_SKIPSPIN kernel option. The flag can also be
set via the loader(8) environment variable
debug.witness.skipspin. If the variable is set to a
non-zero value, then spin mutexes are skipped. Once the kernel has booted,
the status of this flag can be examined but not set via the read-only sysctl
debug.witness.skipspin.
The sysctl debug.witness.watch specifies the level of witness involvement in the system. A value of 1 specifies that witness is enabled. A value of 0 specifies that witness is disabled, but that can be enabled again. This will maintain a small amount of overhead in the system. A value of -1 specifies that witness is disabled permanently and cannot be enabled again. The sysctl debug.witness.watch can be set via loader(8).
The sysctl debug.witness.output_channel
specifies the output channel used to display warnings emitted by
witness. The possible values are
‘console’, indicating that warnings
are to be printed to the system console,
‘log’, indicating that warnings are to
be logged via log(9), and ‘none’.
This sysctl can be set via
loader(8).
The witness code also provides three extra
ddb(4) commands if both witness and
ddb(4) are compiled into the kernel:
show locks[thread]- Outputs the list of locks held by a thread to the kernel console along with the filename and line number at which each lock was last acquired by the thread. The optional thread argument may be either a TID, PID, or pointer to a thread structure. If thread is not specified, then the locks held by the current thread are displayed.
show all locks- Outputs the list of locks held by all threads in the system to the kernel console.
show witness- Dump the current order list to the kernel console. The code first displays the lock order tree for all of the sleep locks. Then it displays the lock order tree for all of the spin locks. Finally, it displays a list of locks that have not yet been acquired.
SEE ALSO
HISTORY
The witness code first appeared in
BSD/OS 5.0 and was imported from there into
FreeBSD 5.0.