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Subject[PATCH 5/8] cgroup: make cgroup hold onto its kernfs_node
Date
cgroup currently releases its kernfs_node when it gets removed.  While
not buggy, this makes cgroup->kn access rules complicated than
necessary and leads to things like get/put protection around
kernfs_remove() in cgroup_destroy_locked(). In addition, we want to
use kernfs_name/path() and friends but also want to be able to
determine a cgroup's name between removal and release.

This patch makes cgroup hold onto its kernfs_node until freed so that
cgroup->kn is always accessible.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
kernel/cgroup.c | 16 +++++++++-------
1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 6da820d..da8aef0 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -957,6 +957,8 @@ static void cgroup_free_fn(struct work_struct *work)

cgroup_pidlist_destroy_all(cgrp);

+ kernfs_put(cgrp->kn);
+
kfree(rcu_dereference_raw(cgrp->name));
kfree(cgrp);
}
@@ -3784,6 +3786,12 @@ static long cgroup_create(struct cgroup *parent, const char *name_str,
}
cgrp->kn = kn;

+ /*
+ * This extra ref will be put in cgroup_free_fn() and guarantees
+ * that @cgrp->kn is always accessible.
+ */
+ kernfs_get(kn);
+
cgrp->serial_nr = cgroup_serial_nr_next++;

/* allocation complete, commit to creation */
@@ -3964,7 +3972,6 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
{
struct cgroup *child;
struct cgroup_subsys_state *css;
- struct kernfs_node *kn;
bool empty;
int ssid;

@@ -4042,13 +4049,8 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
* clearing of cgrp->kn->priv backpointer, which should happen
* after all files under it have been removed.
*/
- kn = cgrp->kn;
- kernfs_get(kn);
-
- kernfs_remove(cgrp->kn);
-
+ kernfs_remove(cgrp->kn); /* @cgrp has an extra ref on its kn */
RCU_INIT_POINTER(*(void __rcu __force **)&cgrp->kn->priv, NULL);
- kernfs_put(kn);

mutex_lock(&cgroup_mutex);

--
1.8.5.3


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