[$] Warning about WARN_ON()

Kernel developers, like conscientious developers for many projects, will
often include checks in the code for conditions that are never expected to
occur, but which would indicate a serious problem should that expectation
turn out to be incorrect. For years, developers have been encouraged (to
put it politely) to avoid using assertions that crash the machine for such
conditions unless there is truly no alternative. Increasingly, though, use
of the kernel's WARN_ON() family of macros, which developers were
told to use instead, is also being discouraged.