  We show by first-principles calculations that the Dirac nodal-line semimetal
phase can co-exist with the ferromagnetic order at room temperature in chromium
dioxide, a widely used material in magnetic tape applications, under small
tensile hydrostatic strains. An ideally flat Dirac nodal ring close to the
Fermi energy is placed in the reflection-invariant boundary of the Brillouin
zone perpendicular to the magnetic order, and is topologically protected by the
unitary mirror symmetry of the magnetic group $D_{4h}(C_{4h})$, which quantizes
the corresponding Berry phase into integer multiples of $\pi$. The
symmetry-dependent topological stability is demonstrated through showing that
only the topologically protected nodal ring can persistently exist under small
anisotropic stains preservingthe symmetry $D_{4h}(C_{4h})$, while the other
seeming band touching points are generically gapped. Our work provides a
practical platform for the investigation of novel physics and potential
applications of the Dirac nodal-line and drumhead fermions, in particular those
related to ferromagnetic properties
