ERROR: Undefined substitution referenced: "LXR".

[dbaluta]

Errors occurs when running:

$ linux/tools/labs$ make docs

/home/daniel/w/repos/linux/Documentation/teaching/labs/deferred_work.rst:721: ERROR: Undefined substitution referenced: "LXR".
/home/daniel/w/repos/linux/Documentation/teaching/labs/deferred_work.rst:721: ERROR: Unknown target name: "lxr".
/home/daniel/w/repos/linux/Documentation/teaching/labs/interrupts.rst:688: ERROR: Undefined substitution referenced: "LXR".
/home/daniel/w/repos/linux/Documentation/teaching/labs/interrupts.rst:688: ERROR: Unknown target name: "lxr".
/home/daniel/w/repos/linux/Documentation/teaching/labs/kernel_api.rst:739: ERROR: Unexpected indentation.
/home/daniel/w/repos/linux/Documentation/teaching/labs/kernel_api.rst:582: ERROR: Undefined substitution referenced: "LXR".
/home/daniel/w/repos/linux/Documentation/teaching/labs/kernel_api.rst:582: ERROR: Unknown target name: "lxr".
/home/daniel/w/repos/linux/Documentation/teaching/labs/kernel_modules.rst:810: ERROR: Undefined substitution referenced: "LXR".
/home/daniel/w/repos/linux/Documentation/teaching/labs/kernel_modules.rst:939: ERROR: Undefined substitution referenced: "LXR".
/home/daniel/w/repos/linux/Documentation/teaching/labs/kernel_modules.rst:810: ERROR: Unknown target name: "lxr".
/home/daniel/w/repos/linux/Documentation/teaching/labs/kernel_modules.rst:939: ERROR: Unknown target name: "lxr".

[dbaluta]

The problem seems to be related with conf.py and subst.rst files.