The -rc kernels released by Linus Torvalds exist for a reason: after 10,000
or so changes flow into the kernel over a two-week merge window, there will
surely be some bugs in need of squashing. The -rc kernels provide an
opportunity for wider testing after all those patches have been
integrated. Most of the time, -rc kernels (even the initial -rc1 releases)
are surprisingly safe to run. Occasionally, though, something goes wrong,
giving early testers reason to reconsider their life choices. The 5.12-rc1
kernel, as it turns out, was one of those.

Source: LWN.net – [$] Linux 5.12’s very bad, double ungood day