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By The Linux Camp News, October 20, 2026
The proposed helper would let common kernel code use two-byte compare-and-exchange on SH, ARC, C-SKY and Xtensa. It does not remove every obstacle to two-byte atomic operations or establish a CPU retirement schedule.
Paul E. McKenney submitted a pull request on October 16, 2026, at 20:51 PDT, targeting Linux 7.4 with two-byte cmpxchg() emulation for SH, ARC, C-SKY and Xtensa. The sources retrieved October 20 establish a submission for the merge window, not confirmed mainline acceptance or availability in a released kernel. 2 1
cmpxchg() means compare-and-exchange. The proposed cmpxchg_emu_u16() helper supplies its two-byte form by using four-byte cmpxchg() underneath. That lets an architecture provide the narrower operation without a native instruction for it. The series connects the helper to the four ports rather than requiring their CPUs to acquire a new hardware capability. The supplied request describes this relationship but does not include the implementation needed to examine alignment, adjacent-byte handling or retries. 1 2
The practical benefit is portability for shared kernel code: McKenney says the change permits two-byte cmpxchg() in common code. For engineers maintaining kernels across these ports, the important distinction is between lacking a native instruction and lacking an implementation that common code can use. This request addresses the latter. It supplies no workload measurements from which to infer an application-speed improvement. 2
Do not treat the helper as blanket support for every two-byte atomic algorithm. McKenney warns that some supported systems lack native two-byte store instructions, so emulating compare-and-exchange still does not make all such algorithms easy to implement. Developers considering a common-code conversion therefore need to distinguish its compare-and-exchange requirement from any separate store requirement. 2
The hardware-support warning is tentative. McKenney anticipates that systems without native two-byte stores may lose Linux support, but the request names neither affected CPU models nor a removal release. It does not establish that SH, ARC, C-SKY or Xtensa will be retired. Operators cannot derive a hardware migration deadline from this proposal. 2