9.9. 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.