Hi Li,
good work in reproducing the issue so quickly!
I have tried to backport this patch to 3.12 kernel but the code has
changed since then.
The only two instances of idr_remove which are called outside of
cgroup_mutex seem to be:
- cgroup_create calling it from err_free_id: path
- css_free_work_fn
mem_cgroup_css_free
__mem_cgroup_free
free_css_id
The second one takes ss->id_lock spinlock which should be sufficient
to exclude get_new_cssid but cgroup_mount and cgroup_create don't use
id_lock. They do hold cgroup_mutex though. So I guess I need something
like the following (I will have it tested):
---
Date: Tue, 11 Feb 2014 16:05:46 +0800
From: Li Zefan <lizefan@xxxxxxxxxx>
Subject: [PATCH - for 3.12] cgroup: protect modifications to cgroup_idr with cgroup_mutex
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 /mnt/sub1/tmp; } &
# for ((; ;)) { mkdir /cgroup/sub2/tmp && rmdir /mnt/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}().
[mhocko@xxxxxxx: ported to 3.12]
Fixes: 4e96ee8e981b ("cgroup: convert cgroup_ida to cgroup_idr")
Cc: <stable@xxxxxxxxxxxxxxx> #3.12+
Reported-by: Michal Hocko <mhocko@xxxxxxx>
Signed-off-by: Li Zefan <lizefan@xxxxxxxxxx>
---
include/linux/cgroup.h | 2 ++
kernel/cgroup.c | 25 ++++++++++++++-----------
2 files changed, 16 insertions(+), 11 deletions(-)
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -169,6 +169,8 @@ 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;
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4410,14 +4410,6 @@ static long cgroup_create(struct cgroup
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)
- goto err_free_name;
-
- /*
* Only live parents can have children. Note that the liveliness
* check isn't strictly necessary because cgroup_mkdir() and
* cgroup_rmdir() are fully synchronized by i_mutex; however, do it
@@ -4426,9 +4418,17 @@ static long cgroup_create(struct cgroup
*/
if (!cgroup_lock_live_group(parent)) {
err = -ENODEV;
- goto err_free_id;
+ goto err_free_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)
+ goto err_unlock;
+
/* 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
@@ -4542,11 +4542,11 @@ err_free_all:
ss->css_free(css);
}
}
+ idr_remove(&root->cgroup_idr, cgrp->id);
+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_free_name:
kfree(rcu_dereference_raw(cgrp->name));
err_free_cgrp:
@@ -5622,9 +5622,12 @@ void free_css_id(struct cgroup_subsys *s
rcu_assign_pointer(id->css, NULL);
rcu_assign_pointer(css->id, NULL);
+
+ mutex_lock(&cgroup_mutex);
spin_lock(&ss->id_lock);
idr_remove(&ss->idr, id->id);
spin_unlock(&ss->id_lock);
+ mutex_unlock(&cgroup_mutex);
kfree_rcu(id, rcu_head);
}
EXPORT_SYMBOL_GPL(free_css_id);
--
Michal Hocko
SUSE Labs
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