[PATCHSET cgroup/for-3.16] cgroup: remove cgroup_tree_mutex

[Tejun Heo <tj@kernel.org>]

Hello,

cgroup_tree_mutex was introduced during kernfs conversion to work
around the cyclic locking dependency between kernfs active protection
and cgroup_mutex.  Some file and directory operations need to acquire
cgroup_mutex which puts the mutex under the kernfs active protection;
however, cgroup also needs to access the hierarchy and the registered
cftypes to detemine which files to remove, which obviously can't be
done while holding cgroup_mutex anymore.

cgroup_tree_mutex nests above both cgroup_mutex and kernfs active
protection and protects hierarchy and cftypes so that those file
operations can be performed while holding it without cgroup_mutex.
This works but is kinda cumbersome as most places end up taking both
cgroup_tree_mutex and cgroup_mutex and there's on-going friction on
what needs to be protected by which combination.

Furthermore, due to new requirements from subtree_control
implementations, kernfs ended up growing full-blown mechanism to
bypass active protection instead of just supporting self-removal and
cgroup ended up using both mechanisms - two layered mutexes and active
protection bypss - on different areas, which is totally unncessary.

This patchset converts everything over to kernfs active protection
bypass and drops cgroup_tree_mutex making cgroup locking noticeably
simpler.  It contains the following eight patches.

 0001-cgroup-reorganize-cgroup_create.patch
 0002-cgroup-collapse-cgroup_create-into-croup_mkdir.patch
 0003-cgroup-grab-cgroup_mutex-earlier-in-cgroup_subtree_c.patch
 0004-cgroup-move-cgroup-kn-priv-clearing-to-cgroup_rmdir.patch
 0005-cgroup-factor-out-cgroup_kn_lock_live-and-cgroup_kn_.patch
 0006-cgroup-use-cgroup_kn_lock_live-in-other-cgroup-kernf.patch
 0007-cgroup-nest-kernfs-active-protection-under-cgroup_mu.patch
 0008-cgroup-remove-cgroup_tree_mutex.patch

0001-0004 reorganize various kernfs handling paths so that they are
more uniform in terms of active protection handling.

0005 factors out two locking helpers - cgroup_kn_lock_live() and
cgroup_kn_unlock() - which handle both kernfs active protection bypass
and locking.

0006 applies it to other kernfs method implementations which were
grabbing cgroup_mutex under active protection.

0007 reverses the locking dependency between cgroup_mutex and kernfs
active protection so that the latter nests under the former, making
cgroup_mutex equivalent to cgroup_tree_mutex.

0008 removes cgroup_tree_mutex.

This patchset is on top of

  cgroup/for-3.16 12d3089c192c ("kernel/cpuset.c: convert printk to pr_foo()")
+ [1] [PATCHSET cgroup/for-3.16] cgroup: post unified hierarchy fixes and updates
+ [2] [PATCHSET cgroup/for-3.16] cgroup: implement cftype->write()

and is available in the following git branch.

 git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git review-kill-tree_mutex

diffstat follows.  Thanks.

 kernel/cgroup.c |  385 +++++++++++++++++++++++---------------------------------
 1 file changed, 163 insertions(+), 222 deletions(-)

--
tejun

[1] http://lkml.kernel.org/g/1399377044-29873-1-git-send-email-tj@kernel.org
[2] http://lkml.kernel.org/g/1399380266-3324-1-git-send-email-tj@kernel.org