Linus has released the 5.1 kernel, right on
schedule. Some of the significant changes in the release include BPF spinlocks, more year-2038 preparation, the TEO CPU-idle governor, The io_uring fast asynchronous I/O mechanism,
initial support for pidfds (file descriptors that refer to a process), the
SafeSetID security module, and much more.
See the KernelNewbies 5.1
page for lots of details.

Source: LWN.net – The 5.1 kernel has been released