NAME

       witness — lock validation facility

SYNOPSIS

       options WITNESS
       options WITNESS_KDB
       options WITNESS_SKIPSPIN

DESCRIPTION

       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_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 that cannot be enabled again.   The  sysctl  debug.witness.watch
       can be set via loader(8).

       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.

SEE ALSO

       ddb(4), loader(8), sysctl(8), mutex(9)

HISTORY

       The witness code first appeared in BSD/OS 5.0 and was imported from there into FreeBSD 5.0.

BUGS

       The witness code currently does not handle recursion of shared sx(9) locks properly.

Debian                                          February 18, 2001                                     WITNESS(4)