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witness (4)
  • >> witness (4) ( FreeBSD man: Специальные файлы /dev/* )

  • BSD mandoc
     

    NAME

    
    
    witness
    
     - lock validation facility
    
     
    

    SYNOPSIS

    options WITNESS options WITNESS_KDB options WITNESS_SKIPSPIN  

    DESCRIPTION

    The 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, 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 code also checks various other conditions such as verifying that one does not recurse on a non-recursive lock. For sleep locks, 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 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. This sysctl can be written to in order to disable witness, however it may not be used to enable witness. The sysctl debug.witness.watch can be set via loader(8).

    The code also provides two extra ddb(4) commands if both 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 code first appeared in Bs x 5.0 and was imported from there into Fx 5.0 .  

    BUGS

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


     

    Index

    NAME
    SYNOPSIS
    DESCRIPTION
    SEE ALSO
    HISTORY
    BUGS


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