From Global to Local Quiescence: Wait-Free Code Patching of Multi-Threaded Processes

 Florian Rommel°, Christian Dietrich°, Daniel Friesel’, Marcel Köppen’, Christoph Borchert’, Michael Müller’, Olaf Spinczyk’, Daniel Lohmann°
°Leibniz Universität Hannover
‘Universität Osnabrück

Abstract

Live patching has become a common technique to keep long-running system services secure and up-to-date without causing downtimes during patch application. However, to safely apply a patch, existing live-update methods require the entire process to enter a state of quiescence, which can be highly disruptive for multi-threaded programs: Having to halt all threads (e.g., at a global barrier) for the patching not only hampers quality of service, but can also be tremendously difficult to get “right” in the implementation without causing deadlocks or other synchronization issues.

In this paper, we present WfPatch, a wait-free approach to inject code changes into running multi-threaded programs. Instead of having to stop the world before applying a patch, WfPatch can gradually apply it to each thread individually at a local point of quiescence, while all other threads can make uninterrupted progress.

We have implemented WfPatch as a kernel service and user-space library for Linux 5.1 and evaluated it with OpenLDAP, Apache, memcached, Samba, Node.js, and MariaDB on Debian 10 (“buster”). In total, we successfully applied 33 different binary patches into running programs while they were actively servicing requests; 15 patches had a CVE number or were other critical updates. Applying a patch with WfPatch did not lead to any noticeable increase in request latencies – even under high load – while applying the same patch after reaching global quiescence increases tail latencies by a factor of up to 41× for MariaDB.