Linux 5.10 contains this patch:

commit 2a36ab717e8fe678d98f81c14a0b124712719840
Author: Peter Oskolkov [off-list ref]
Date:   Wed Sep 23 16:36:16 2020 -0700

    rseq/membarrier: Add MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ

This adds an argument to an existing syscall.  Before the patch,
membarrier had 2 parameters; now it has 3.  Is this really okay?  At
least the patch is careful and ignores the third parameter unless a
previously unused flag bit is set.

--Andy