10.9 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 allpcpu, show alllocks, show lockedvnods and show alltrace.

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