[$] Restartable sequences in glibc

"Restartable sequences" are small segments of user-space code designed toaccess per-CPU data structures without the need for heavyweight locking.It is a relatively obscure feature, despite having been supported by theLinux kernel since the 4.18 release. Among other things, there is nosupport in the GNU C Library (glibc) for this feature. That is about tochange with the upcoming glibc 2.35release, though, so a look at the user-space APIfor this feature is warranted.