1. 21 Jul, 2012 30 commits
  2. 11 Jul, 2012 9 commits
  3. 07 Jul, 2012 1 commit
    • Tejun Heo's avatar
      cgroup: fix cgroup hierarchy umount race · 5db9a4d9
      Tejun Heo authored
      48ddbe19 "cgroup: make css->refcnt clearing on cgroup removal
      optional" allowed a css to linger after the associated cgroup is
      removed.  As a css holds a reference on the cgroup's dentry, it means
      that cgroup dentries may linger for a while.
      
      Destroying a superblock which has dentries with positive refcnts is a
      critical bug and triggers BUG() in vfs code.  As each cgroup dentry
      holds an s_active reference, any lingering cgroup has both its dentry
      and the superblock pinned and thus preventing premature release of
      superblock.
      
      Unfortunately, after 48ddbe19, there's a small window while
      releasing a cgroup which is directly under the root of the hierarchy.
      When a cgroup directory is released, vfs layer first deletes the
      corresponding dentry and then invokes dput() on the parent, which may
      recurse further, so when a cgroup directly below root cgroup is
      released, the cgroup is first destroyed - which releases the s_active
      it was holding - and then the dentry for the root cgroup is dput().
      
      This creates a window where the root dentry's refcnt isn't zero but
      superblock's s_active is.  If umount happens before or during this
      window, vfs will see the root dentry with non-zero refcnt and trigger
      BUG().
      
      Before 48ddbe19, this problem didn't exist because the last dentry
      reference was guaranteed to be put synchronously from rmdir(2)
      invocation which holds s_active around the whole process.
      
      Fix it by holding an extra superblock->s_active reference across
      dput() from css release, which is the dput() path added by 48ddbe19
      and the only one which doesn't hold an extra s_active ref across the
      final cgroup dput().
      Signed-off-by: default avatarTejun Heo <tj@kernel.org>
      LKML-Reference: <4FEEA5CB.8070809@huawei.com>
      Reported-by: default avatarshyju pv <shyju.pv@huawei.com>
      Tested-by: default avatarshyju pv <shyju.pv@huawei.com>
      Cc: Sasha Levin <levinsasha928@gmail.com>
      Acked-by: default avatarLi Zefan <lizefan@huawei.com>
      5db9a4d9