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cgroup: break kernfs active_ref protection in cgroup directory operations 

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@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Tejun Heo 2014-03-20 11:10:15 -04:00
parent 1b9aba49ea
commit e1b2dc176f
1 changed files with 26 additions and 1 deletions
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27
kernel/cgroup.c
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@ -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);
mutex_lock(&cgroup_tree_mutex);
mutex_lock(&cgroup_mutex);
@ -2331,6 +2339,9 @@ static int cgroup_rename(struct kernfs_node *kn, struct kernfs_node *new_parent,
mutex_unlock(&cgroup_mutex);
mutex_unlock(&cgroup_tree_mutex);
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;
}
/*