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witness - lock validation facility



      witness - lock validation facility


      options WITNESS
      options WITNESS_KDB
      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 debug‐
      ger 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 speci‐
      fied, 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 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.  This sysctl can be written to
      in order to disable witness, however it may not be used to enable wit‐
      ness.  The sysctl 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 con‐
      sole 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 dis‐
      plays 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), loader(8), sysctl(8), 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.


Based on BSD UNIX
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