From 0ab02ca8f887908152d1a96db5130fc661d36a1e Mon Sep 17 00:00:00 2001 From: Li Zefan Date: Tue, 11 Feb 2014 16:05:46 +0800 Subject: [PATCH] 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: [] dump_stack+0x7a/0x96 [] warn_slowpath_common+0x8c/0xc0 [] warn_slowpath_fmt+0x46/0x50 [] sub_remove+0x87/0x1b0 [] ? css_killed_work_fn+0x32/0x1b0 [] idr_remove+0x25/0xd0 [] cgroup_destroy_css_killed+0x5b/0xc0 [] css_killed_work_fn+0x130/0x1b0 [] process_one_work+0x26c/0x550 [] worker_thread+0x12e/0x3b0 [] kthread+0xe6/0xf0 [] 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}(). tj: Refreshed on top of b58c89986a77 ("cgroup: fix error return from cgroup_create()"). Fixes: 4e96ee8e981b ("cgroup: convert cgroup_ida to cgroup_idr") Cc: #3.12+ Reported-by: Michal Hocko Signed-off-by: Li Zefan Signed-off-by: Tejun Heo --- include/linux/cgroup.h | 2 ++ kernel/cgroup.c | 34 ++++++++++++++++++---------------- 2 files changed, 20 insertions(+), 16 deletions(-) diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h index 5c097596104b..9450f025fe0c 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 3edf7163b84f..52719ce55dd3 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); @@ -4167,16 +4169,6 @@ 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; - } - /* * Only live parents can have children. Note that the liveliness * check isn't strictly necessary because cgroup_mkdir() and @@ -4186,7 +4178,17 @@ 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; + } + + /* + * 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; } /* Grab a reference on the superblock so the hierarchy doesn't @@ -4218,7 +4220,7 @@ 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++; @@ -4254,12 +4256,12 @@ 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); + /* Release the reference count that we took on the superblock */ + deactivate_super(sb); +err_unlock: + mutex_unlock(&cgroup_mutex); err_free_name: kfree(rcu_dereference_raw(cgrp->name)); err_free_cgrp: -- 2.20.1