Updates to sys_membarrier for v4.14 to add registration

Hello!

This post-merge-window series fixes a problem that arose when designing requested extensions to sys_membarrier() to allow JITs to efficiently flush old code from instruction caches. Several potential algorithms are much less painful if the user register intent to use this functionality early on, for example, before the process spawns the second thread. Registering at this time removes the need to interrupt each and every thread in that process at the first expedited sys_membarrier() system call.

However, if we let the current function out, then user programs might be written without registration, which would make it more difficult to add required registration after the fact. Hence the late-in-merge-window request. (Ingo, pull request on its way as well.)

1. Provide register expedited private command.

2. Test private expedited cmd.

3. Document scheduler barrier requirements.

All courtesy of Mathieu Desnoyers.

Thanx, Paul

------------------------------------------------------------------------

 MAINTAINERS                                          |    2 
 arch/powerpc/Kconfig                                 |    1 
 arch/powerpc/include/asm/membarrier.h                |   43 +++++++
 arch/powerpc/include/asm/thread_info.h               |    3 
 arch/powerpc/kernel/Makefile                         |    2 
 arch/powerpc/kernel/membarrier.c                     |   45 +++++++
 arch/powerpc/mm/mmu_context.c                        |    7 +
 arch/x86/mm/tlb.c                                    |    5 
 fs/exec.c                                            |    1 
 include/linux/mm_types.h                             |    3 
 include/linux/sched/mm.h                             |   55 +++++++++
 include/uapi/linux/membarrier.h                      |   23 ++--
 init/Kconfig                                         |    3 
 kernel/fork.c                                        |    2 
 kernel/sched/core.c                                  |   47 ++++----
 kernel/sched/membarrier.c                            |   25 +++-
 tools/testing/selftests/membarrier/membarrier_test.c |  109 ++++++++++++++++---
 17 files changed, 329 insertions(+), 47 deletions(-)