Michael Sullivan proposed a clever hack abusing mprotect() to
perform the same effect as sys_membarrier() I submitted a few
years ago ( https://lkml.org/lkml/2010/4/18/15 ). 

At that time, the sys_membarrier implementation was deemed
technically sound, but there were not enough users of the system call
to justify its inclusion. 

So far, the number of users of liburcu has increased, but liburcu
still appears to be the only direct user of sys_membarrier.  On this
front, we could argue that many other system calls have only
one user: glibc.  In that respect, liburcu is quite similar to glibc. 

So the question as it stands appears to be: would you be comfortable
having users abuse mprotect(), relying on its side-effect of issuing
a smp_mb() on each targeted CPU for the TLB shootdown, as
an effective implementation of process-wide memory barrier ?