[no subject]

From: Tejun Heo
Date: Thu Mar 20 2014 - 11:10:15 EST

cgroup_tree_mutex should nest above the kernfs active_ref protection;
however, cgroup_create() and cgroup_rename() were grabbing
cgroup_tree_mutex while under kernfs active_ref protection. This has
actualy possibility to lead to deadlocks in case these operations race
against cgroup_rmdir() which invokes kernfs_remove() on directory
kernfs_node while holding cgroup_tree_mutex.

Neither cgroup_create() or cgroup_rename() requires active_ref
protection. The former already has enough synchronization through
cgroup_lock_live_group() and the latter doesn't care, so this can be
fixed by updating both functions to break all active_ref protections
before grabbing cgroup_tree_mutex.

While this patch fixes the immediate issue, it probably needs further
work in the long term - kernfs directories should enable lockdep
annotations and maybe the better way to handle this is marking
directory nodes as not needing active_ref protection rather than
breaking it in each operation.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

Another last minute fix that I just committed. I'm thinking maybe the
right thing to do for cgroup directories is implementing a flag
similar KERNFS_NO_ACTIVE_REF, which achieves similar thing but doesn't
require each operation to break protection explicitly. Anyways, this
should be enough for now.


kernel/cgroup.c | 27 ++++++++++++++++++++++++++-
1 file changed, 26 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 98a8045..58c67b3 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2324,6 +2324,14 @@ static int cgroup_rename(struct kernfs_node *kn, struct kernfs_node *new_parent,
if (cgroup_sane_behavior(cgrp))
return -EPERM;

+ /*
+ * We're gonna grab cgroup_tree_mutex which nests outside kernfs
+ * active_ref. kernfs_rename() doesn't require active_ref
+ * protection. Break them before grabbing cgroup_tree_mutex.
+ */
+ kernfs_break_active_protection(new_parent);
+ kernfs_break_active_protection(kn);

@@ -2331,6 +2339,9 @@ static int cgroup_rename(struct kernfs_node *kn, struct kernfs_node *new_parent,

+ kernfs_unbreak_active_protection(kn);
+ kernfs_unbreak_active_protection(new_parent);
return ret;

@@ -3778,8 +3789,22 @@ static int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name,
umode_t mode)
struct cgroup *parent = parent_kn->priv;
+ int ret;

- return cgroup_create(parent, name, mode);
+ /*
+ * cgroup_create() grabs cgroup_tree_mutex which nests outside
+ * kernfs active_ref and cgroup_create() already synchronizes
+ * properly against removal through cgroup_lock_live_group().
+ * Break it before calling cgroup_create().
+ */
+ cgroup_get(parent);
+ kernfs_break_active_protection(parent_kn);
+ ret = cgroup_create(parent, name, mode);
+ kernfs_unbreak_active_protection(parent_kn);
+ cgroup_put(parent);
+ return ret;


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