rseq(2)

Purpose

rseq() registers a restartable sequence area for the calling thread: a userspace-visible struct rseq (plus an optional struct rseq_cs critical-section descriptor) that the kernel keeps coherent across preemption and migration. Code inside a declared critical section either runs to completion or the kernel aborts it by writing the abort tag into the area, giving lock-free algorithms a cheap fallback path without CAS loops on shared flags (kernel/rseq.c; /usr/include/linux/rseq.h). It is the interface behind per-CPU counters, resizable lock-free structures, and glibc/TLS fast paths that must not pay an atomic per access.

Synopsis

#include <linux/rseq.h>   /* struct rseq, RSEQ_FLAG_* (uapi header) */
long syscall(SYS_rseq, struct rseq *rseq, uint32_t rseq_len,
             int flags, uint32_t sig);

Arguments and behavior

rseq points to the thread’s area (32-byte aligned in practice via the ABI), rseq_len is its length (must match sizeof(struct rseq)), sig is a 32-bit signature written into abort events so userspace can distinguish aborts from its own code, and flags selects the operation (kernel/rseq.c, SYSCALL_DEFINE4(rseq)). Registration uses flags 0; unregistering passes RSEQ_FLAG_UNREGISTER (value 1 << 0, /usr/include/linux/rseq.h enum). Re-registering without unregistering fails with EBUSY; registering with the wrong length fails with EINVAL; a mismatched sig on re-registration fails with EPERM (kernel/rseq.c, rseq registration checks). The kernel updates cpu_id, node_id, mm_cid and the critical-section state on every return to userspace; on abort it sets rseq_cs to the signature ORed with the abort flag bits (RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT/SIGNAL/MIGRATE select which events abort, /usr/include/linux/rseq.h).

Return value and errors

0 on success; a negated error on the raw path. Documented in the source: EINVAL (bad flags, wrong rseq_len, unaligned area), EFAULT (area unreadable), EBUSY (already registered), EPERM (sig mismatch on re-registration), ENOSYS on kernels without CONFIG_RSEQ (kernel/rseq.c).

Flags and constants

Security and notes

One registered area per thread; the area is per-thread state even though the struct lives in shared memory by convention. The signature (sig) exists so a jumped-to abort handler can verify the abort came from the expected registration – a hardening against stray memory corruption rather than a security boundary. glibc 2.35+ registers the area itself; libraries must detect and reuse the registered area (the __rseq symbol) instead of calling rseq() again, or they will hit EBUSY. The 6.x-era follow-up syscalls in this family: rseq_slice_yield (see rseq_slice_yield). This page is source-limited: kernel/rseq.c (master, fall 2026) and the uapi header were the reachable sources; no man page is hosted on man7.org. Cross-reference: futex, getcpu, sched_setaffinity, membarrier.

References

• Kernel source: kernel/rseq.c (SYSCALL_DEFINE4(rseq), torvalds/linux master, fetched 2026-10-17)
• UAPI header: /usr/include/linux/rseq.h (struct rseq, RSEQ_FLAG_UNREGISTER, RSEQ_CS_FLAG_*)
• man-pages syscalls(2) version table (4.18)
• worklog: no exported rseq() wrapper symbol (nm -D, glibc 2.41)