Restartable Sequences (rseq) is a Linux kernel feature that enables lock-free, atomic operations without expensive system calls. The mechanism allows userspace code to execute critical sections that automatically restart if interrupted.
Restartable Sequences solve a fundamental problem in concurrent programming: protecting shared data without heavy locks. When a CPU core is interrupted during an rseq-protected section, the kernel automatically rolls back execution to the start, avoiding data corruption.
The feature works by registering a code region with the kernel. If preemption occurs mid-execution, control returns to the sequence's beginning rather than resuming mid-operation. This eliminates the need for expensive synchronization primitives in hot paths.
Applications include thread-local storage updates, memory allocation, and task scheduling. Userspace runtimes like glibc and Java have adopted rseq for performance-critical operations. The technique reduces context-switch overhead and improves scalability on high-core-count systems.
Rseq landed in Linux 4.18 (2018) but remains underutilized. Recent discussions highlight its potential for improving performance in databases, language runtimes, and system software where traditional locking creates bottlenecks.
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