FreeBSD Developers' Handbook
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# 10.7 Debugging Deadlocks

You may experience so called deadlocks, the situation where a system stops doing useful work. To provide a helpful bug report in this situation, use ddb(4) as described above. Include the output of `ps` and `trace` for suspected processes in the report.

If possible, consider doing further investigation. The receipt below is especially useful if you suspect that a deadlock occurs in the VFS layer. Add the following options

    makeoptions		DEBUG=-g
    	options		INVARIANTS
    	options		INVARIANT_SUPPORT
    	options		WITNESS
    	options		DEBUG_LOCKS
    	options		DEBUG_VFS_LOCKS
    	options		DIAGNOSTIC

to the kernel configuration file. When a deadlock occurs, in addition to the output of the `ps` command, provide information from the `show pcpu`, `show allpcpu`, `show locks`, `show alllocks`, `show lockedvnods` and `alltrace`.

To obtain meaningful backtraces for threaded processes, use `thread thread-id` to switch to the thread stack, and do a backtrace with `where`.

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