Setup cgroupfs like this:
# mount -t cgroup -o cpuacct xxx /cgroup
# mkdir /cgroup/sub1
# mkdir /cgroup/sub2
Then run these two commands:
# for ((; ;)) { mkdir /cgroup/sub1/tmp && rmdir /cgroup/sub1/tmp; } &
# for ((; ;)) { mkdir /cgroup/sub2/tmp && rmdir /cgroup/sub2/tmp; } &
After seconds you may see this warning:
------------[ cut here ]------------
WARNING: CPU: 1 PID: 25243 at lib/idr.c:527 sub_remove+0x87/0x1b0()
idr_remove called for id=6 which is not allocated.
...
Call Trace:
[<ffffffff8156063c>] dump_stack+0x7a/0x96
[<ffffffff810591ac>] warn_slowpath_common+0x8c/0xc0
[<ffffffff81059296>] warn_slowpath_fmt+0x46/0x50
[<ffffffff81300aa7>] sub_remove+0x87/0x1b0
[<ffffffff810f3f02>] ? css_killed_work_fn+0x32/0x1b0
[<ffffffff81300bf5>] idr_remove+0x25/0xd0
[<ffffffff810f2bab>] cgroup_destroy_css_killed+0x5b/0xc0
[<ffffffff810f4000>] css_killed_work_fn+0x130/0x1b0
[<ffffffff8107cdbc>] process_one_work+0x26c/0x550
[<ffffffff8107eefe>] worker_thread+0x12e/0x3b0
[<ffffffff81085f96>] kthread+0xe6/0xf0
[<ffffffff81570bac>] ret_from_fork+0x7c/0xb0
---[ end trace 2d1577ec10cf80d0 ]---
It's because allocating/removing cgroup ID is not properly synchronized.
The bug was introduced when we converted cgroup_ida to cgroup_idr.
While synchronization is already done inside ida_simple_{get,remove}(),
users are responsible for concurrent calls to idr_{alloc,remove}().
v2:
- Don't call deactivate_super() inside cgroup_mutex, as cgroup_kill_sb()
will be called if sb refcnt reaches 0. I don't think this can happen,
as cgroup_create() is called through vfs, so vfs should guarantee the
superblock won't disappear. Still better not depend on it even my guess
is probably correct.
Fixes: 4e96ee8e981b ("cgroup: convert cgroup_ida to cgroup_idr")
Reported-by: Michal Hocko <mhocko@xxxxxxx>
Signed-off-by: Li Zefan <lizefan@xxxxxxxxxx>
---
Also rebased on "cgroup: fix error return from cgroup_create()".
---
include/linux/cgroup.h | 2 ++
kernel/cgroup.c | 47 ++++++++++++++++++++++++++---------------------
2 files changed, 28 insertions(+), 21 deletions(-)
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 5c09759..9450f02 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -166,6 +166,8 @@ struct cgroup {
*
* The ID of the root cgroup is always 0, and a new cgroup
* will be assigned with a smallest available ID.
+ *
+ * Allocating/Removing ID must be protected by cgroup_mutex.
*/
int id;
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 7be8fe4..37f217c 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -886,7 +886,9 @@ static void cgroup_diput(struct dentry *dentry, struct inode *inode)
* per-subsystem and moved to css->id so that lookups are
* successful until the target css is released.
*/
+ mutex_lock(&cgroup_mutex);
idr_remove(&cgrp->root->cgroup_idr, cgrp->id);
+ mutex_unlock(&cgroup_mutex);
cgrp->id = -1;
call_rcu(&cgrp->rcu_head, cgroup_free_rcu);
@@ -4174,15 +4176,12 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
}
rcu_assign_pointer(cgrp->name, name);
- /*
- * Temporarily set the pointer to NULL, so idr_find() won't return
- * a half-baked cgroup.
- */
- cgrp->id = idr_alloc(&root->cgroup_idr, NULL, 1, 0, GFP_KERNEL);
- if (cgrp->id < 0) {
- err = -ENOMEM;
- goto err_free_name;
- }
+ /* Grab a reference on the superblock so the hierarchy doesn't
+ * get deleted on unmount if there are child cgroups. This
+ * can be done outside cgroup_mutex, since the sb can't
+ * disappear while someone has an open control file on the
+ * fs */
+ atomic_inc(&sb->s_active);
/*
* Only live parents can have children. Note that the liveliness
@@ -4193,15 +4192,18 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
*/
if (!cgroup_lock_live_group(parent)) {
err = -ENODEV;
- goto err_free_id;
+ goto err_free_name;
}
- /* Grab a reference on the superblock so the hierarchy doesn't
- * get deleted on unmount if there are child cgroups. This
- * can be done outside cgroup_mutex, since the sb can't
- * disappear while someone has an open control file on the
- * fs */
- atomic_inc(&sb->s_active);
+ /*
+ * Temporarily set the pointer to NULL, so idr_find() won't return
+ * a half-baked cgroup.
+ */
+ cgrp->id = idr_alloc(&root->cgroup_idr, NULL, 1, 0, GFP_KERNEL);
+ if (cgrp->id < 0) {
+ err = -ENOMEM;
+ goto err_unlock;
+ }
init_cgroup_housekeeping(cgrp);
@@ -4225,7 +4227,7 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
*/
err = cgroup_create_file(dentry, S_IFDIR | mode, sb);
if (err < 0)
- goto err_unlock;
+ goto err_free_id;
lockdep_assert_held(&dentry->d_inode->i_mutex);
cgrp->serial_nr = cgroup_serial_nr_next++;
@@ -4261,13 +4263,16 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
return 0;
-err_unlock:
- mutex_unlock(&cgroup_mutex);
- /* Release the reference count that we took on the superblock */
- deactivate_super(sb);
err_free_id:
idr_remove(&root->cgroup_idr, cgrp->id);
+err_unlock:
+ mutex_unlock(&cgroup_mutex);
err_free_name:
+ /*
+ * Release the reference count that we took on the superblock.
+ * Should be called outside cgroup_mutex.
+ */
+ deactivate_super(sb);
kfree(rcu_dereference_raw(cgrp->name));
err_free_cgrp:
kfree(cgrp);
--
1.8.0.2
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