[$] Restartable sequences and ops vectors

Some technologies find their way into the kernel almost immediately; othersneed to go through multiple iterations over a number of years first.Restartable sequences, a mechanism for lockless concurrency control in user space, fallinto the latter category. At the 2017 Kernel Summit, Mathieu Desnoyersdiscussed yet another implementation of this concept - but this one may notbe the last word either.