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SubjectRe: [PATCH 04/17] cgroup: create directory before linking while creating a new cgroup
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 <tj@kernel.org>
> ---
> kernel/cgroup.c | 32 +++++++++++++-------------------
> 1 file changed, 13 insertions(+), 19 deletions(-)



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