witness -- lock validation facility
options WITNESS
options WITNESS_DDB
options WITNESS_SKIPSPIN
The witness 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 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. For sleep locks,
witness verifies that a new process would not be switched to when a lock
is released or a lock is blocked on during an acquire while any spin
locks are held. If any of these checks fail, then the kernel will panic.
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_DDB kernel option is specified,
then the flag will default to on. It can also be set from the
loader(8) via the debug.witness_ddb environment variable or after the
kernel has booted via the debug.witness_ddb 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 witness code also provides two extra ddb(4) commands if both witness
and ddb(4) are compiled into the kernel:
show locks
Outputs the list of locks held by the current thread to the kernel console
along with the filename and line number at which each lock was last
acquired by this thread.
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.
ddb(4), mutex(9)
The witness code first appeared in BSD/OS 5.0 and was imported from there
into FreeBSD 5.0.
The witness code currently does not handle recursion of shared sx(9)
locks properly.
FreeBSD 5.2.1 February 18, 2001 FreeBSD 5.2.1 [ Back ] |
