The 5.3 kernel is available at last. The
announcement includes a long discussion about user-space regressions — an
ext4 filesystem performance improvement had caused some systems to fail
booting due to a lack of entropy early after startup. “It’s more
that it’s an instructive example of what counts as a regression, and what
the whole ‘no regressions’ kernel rule means. The reverted commit didn’t
change any API’s, and it didn’t introduce any new bugs. But it ended up
exposing another problem, and as such caused a kernel upgrade to fail for a
user. So it got reverted.”

Some of the more significant changes in 5.3 include
scheduler utilization clamping,
the pidfd_open() and
clone3() system calls,
bounded loop support for BPF programs,
support for the 0.0.0.0/8 IPv4 address range,
a new configuration
option for the soon-to-be-merged realtime preemption code,
and more. See the
KernelNewbies 5.3 page for lots of details.

Source: LWN.net – The 5.3 kernel is out