[$] Extending restartable sequences with virtual CPU IDs

Restartable sequences, a Linux kernel feature that facilitates the writingof lockless, per-CPU code in user space, has been around for some years,but it only just received support in the GNU CLibrary this month. Now that this barrier has been crossed, it wouldseem that the time has come to start adding features. Mathieu Desnoyershas responded to this challenge with apatch set adding an extension mechanism and a new "virtual CPU ID"feature.