The membarrier syscall was added in kernel 4.3 by the following kernel commit:

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commit 5b25b13ab08f616efd566347d809b4ece54570d1                                 
Author: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>                      
Date:   Fri Sep 11 13:07:39 2015 -0700                                          
                                                                                
    sys_membarrier(): system-wide memory barrier (generic, x86)                 
                                                                                
    Here is an implementation of a new system call, sys_membarrier(), which     
    executes a memory barrier on all threads running on the system.  It is      
    implemented by calling synchronize_sched().  It can be used to              
    distribute the cost of user-space memory barriers asymmetrically by         
    transforming pairs of memory barriers into pairs consisting of              
    sys_membarrier() and a compiler barrier.  For synchronization primitives    
    that distinguish between read-side and write-side (e.g.  userspace RCU      
    [1], rwlocks), the read-side can be accelerated significantly by moving     
    the bulk of the memory barrier overhead to the write-side.                  
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Support for membarrier() should be added to the syscall tapset.