[RFC][PATCH 00/13] percpu rwsem -v2

[Peter Zijlstra]

This is a derived work of the cpu hotplug lock rework I did in 2013 which never
really went anywhere because Linus didn't like it.

This applies those same optimizations to the percpu-rwsem. Seeing how we did
all the work it seemed a waste to not use it at all. Linus still didn't like it
because there was only a single user, there are two now:

 - uprobes
 - cgroups

This series converts the cpu hotplug lock into a percpu-rwsem to provide a 3rd
user.

Also, since Linus thinks lglocks is a failed locking primitive (which I whole
heartedly agree with, its preempt-disable latencies are an abomination), it
also converts the global part of fs/locks's usage of lglock over to a
percpu-rwsem and uses a per-cpu spinlock for the local part. This both provides
another (4th) percpu-rwsem users and removes an lglock user.

It further removes the stop_machine lglock usage, and with it kills lglocks.

Changes since -v1:

 - Added the missing smp_load_acquire()/smp_store_release() as spotted by Oleg
 - Added percpu_down_read_trylock()
 - Convert cpu hotplug lock
 - Convert fs/locks
 - Removes lglock from stop_machine
 - Removes lglock

---
 Documentation/locking/lglock.txt | 166 -------------------------
 fs/Kconfig                       |   1 +
 fs/file_table.c                  |   1 -
 fs/locks.c                       |  65 +++++++---
 include/linux/cpu.h              |   6 +
 include/linux/lglock.h           |  81 -------------
 include/linux/percpu-rwsem.h     |  96 +++++++++++++--
 include/linux/sched.h            |   9 +-
 init/main.c                      |   1 +
 kernel/cpu.c                     | 130 ++++++--------------
 kernel/fork.c                    |   2 +
 kernel/locking/Makefile          |   1 -
 kernel/locking/lglock.c          | 111 -----------------
 kernel/locking/percpu-rwsem.c    | 255 +++++++++++++++++++++------------------
 kernel/rcu/Makefile              |   2 +-
 kernel/stop_machine.c            |  52 ++++----
 lib/Kconfig                      |  10 ++
 17 files changed, 371 insertions(+), 618 deletions(-)

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