FreeBSD Developers' Handbook
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# 10.9 Debugging the Deadlocks

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

If possible, consider doing further investigation. Receipt below is especially useful if you suspect deadlock occurs in the VFS layer. Add the options

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

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

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

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