Merge branch 'for-3.5-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup
Pull cgroup fixes from Tejun Heo:
"The previous cgroup pull request contained a patch to fix a race
condition during cgroup hierarchy umount. Unfortunately, while the
patch reduced the race window such that the test case I and Sasha were
using didn't trigger it anymore, it wasn't complete - Shyju and Li
could reliably trigger the race condition using a different test case.
The problem wasn't the gap between dentry deletion and release which
the previous patch tried to fix. The window was between the last
dput() of a root's child and the resulting dput() of the root. For
cgroup dentries, the deletion and release always happen synchronously.
As this releases the s_active ref, the refcnt of the root dentry,
which doesn't hold s_active, stays above zero without the
corresponding s_active. If umount was in progress, the last
deactivate_super() proceeds to destory the superblock and triggers
BUG() on the non-zero root dentry refcnt after shrinking.
This issue surfaced because cgroup dentries are now allowed to linger
after rmdir(2) since 3.5-rc1. Before, rmdir synchronously drained the
dentry refcnt and the s_active acquired by rmdir from vfs layer
protected the whole thing. After 3.5-rc1, cgroup may internally hold
and put dentry refs after rmdir finishes and the delayed dput()
doesn't have surrounding s_active ref exposing this issue.
This pull request contains two patches - one reverting the previous
incorrect fix and the other adding the surrounding s_active ref around
the delayed dput().
This is quite late in the release cycle but the change is on the safer
side and fixes the test cases reliably, so I don't think it's too
crazy."
* 'for-3.5-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup:
cgroup: fix cgroup hierarchy umount race
Revert "cgroup: superblock can't be released with active dentries"
