On 2014/5/7 4:19, Tejun Heo wrote:
> 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(-)
>
Acked-by: Li Zefan <lizefan@xxxxxxxxxx>
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