NAME

       witness — lock validation facility

SYNOPSIS

       options WITNESS
       options WITNESS_COUNT
       options WITNESS_KDB
       options WITNESS_NO_VNODE
       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 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

       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.

Debian                                          November 18, 2015                                     WITNESS(4)