On 2012/11/13 11:01, Tejun Heo wrote:

While creating a new cgroup, cgroup_create() links the newly allocated
cgroup into various places before trying to create its directory.
Because cgroup life-cycle is tied to the vfs objects, this makes it
impossible to use cgroup_rmdir() for rolling back creation - the
removal logic depends on having full vfs objects.

This patch moves directory creation above linking and collect linking
operations to one place.  This allows directory creation failure to
share error exit path with css allocation failures and any failure
sites afterwards (to be added later) can use cgroup_rmdir() logic to
undo creation.

It also removes the need to check whether cgroup->dentry is %NULL in
cgroup_path.  If a cgroup is visible, its dentry is guaranteed to be
visible too.

I'm afraid this isn't safe.

The cgroup is visible to a controller soon after ss->create(), and then
the controller might call cgroup_path() while cgrp->dentry is still NULL.

This can happen for cpu cgroup I think, and in fact the NULL check was
added for this reason IIRC.

Note that this also makes the memory barriers around cgroup->dentry,
which currently is misleadingly using RCU operations, unnecessary.
This will be handled in the next patch.

While at it, locking BUG_ON() on i_mutex is converted to
lockdep_assert_held().

Signed-off-by: Tejun Heo <redacted>
---
 kernel/cgroup.c | 32 +++++++++++++-------------------
 1 file changed, 13 insertions(+), 19 deletions(-)