[PATCH v2 0/8] Add rcu_sync infrastructure to avoid _expedited() in percpu-rwsem

[Oleg Nesterov <oleg@redhat.com>]

Hello,

Now that sb->s_writers was changed to use percpu_rw_semaphore let me
send v2.

Changes:

	- whitespace fix in 1/8.

	- remove EXPORT_SYMBOL() in 3/8, currently rcu_sync has no
	  modular users.

	- 5/8 is new. This ugly hack pairs with another one:
	  "shift percpu_counter_destroy() into destroy_super_work()"
	  https://git.kernel.org/cgit/linux/kernel/git/viro/vfs.git/commit/?h=for-next&id=853b39a7c82826b8413048feec7bf08e98ce7a84
	  They both will be reverted later.

	  The problem is that we have 2 series routed via different
	  trees, RCU and VFS. We need this hack to ensure that this
	  series won't break alloc_super() which currently assumes that
	  destroy_super()->percpu_free_rwsem() is safe after kzalloc().
	  This way these 2 series do not depend on each other, we can
	  test/change/revert/etc them independently.

	  We will add rcu_sync_dtor() into deactivate_locked_super()
	  later and revert both hacks.
Oleg.

 include/linux/percpu-rwsem.h  |    3 +-
 include/linux/rcusync.h       |   56 +++++++++++++++
 kernel/locking/percpu-rwsem.c |   85 ++++++++---------------
 kernel/rcu/Makefile           |    2 +-
 kernel/rcu/sync.c             |  151 +++++++++++++++++++++++++++++++++++++++++
 5 files changed, 240 insertions(+), 57 deletions(-)