This is a note to let you know that I've just added the patch titled cgroup: avoid attaching a cgroup root to two different superblocks to the 3.18-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: cgroup-avoid-attaching-a-cgroup-root-to-two-different-superblocks.patch and it can be found in the queue-3.18 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From bfb0b80db5f9dca5ac0a5fd0edb765ee555e5a8e Mon Sep 17 00:00:00 2001 From: Zefan Li <lizefan@xxxxxxxxxx> Date: Fri, 7 Apr 2017 16:51:55 +0800 Subject: cgroup: avoid attaching a cgroup root to two different superblocks From: Zefan Li <lizefan@xxxxxxxxxx> commit bfb0b80db5f9dca5ac0a5fd0edb765ee555e5a8e upstream. Run this: touch file0 for ((; ;)) { mount -t cpuset xxx file0 } And this concurrently: touch file1 for ((; ;)) { mount -t cpuset xxx file1 } We'll trigger a warning like this: ------------[ cut here ]------------ WARNING: CPU: 1 PID: 4675 at lib/percpu-refcount.c:317 percpu_ref_kill_and_confirm+0x92/0xb0 percpu_ref_kill_and_confirm called more than once on css_release! CPU: 1 PID: 4675 Comm: mount Not tainted 4.11.0-rc5+ #5 Hardware name: Bochs Bochs, BIOS Bochs 01/01/2007 Call Trace: dump_stack+0x63/0x84 __warn+0xd1/0xf0 warn_slowpath_fmt+0x5f/0x80 percpu_ref_kill_and_confirm+0x92/0xb0 cgroup_kill_sb+0x95/0xb0 deactivate_locked_super+0x43/0x70 deactivate_super+0x46/0x60 ... ---[ end trace a79f61c2a2633700 ]--- Here's a race: Thread A Thread B cgroup1_mount() # alloc a new cgroup root cgroup_setup_root() cgroup1_mount() # no sb yet, returns NULL kernfs_pin_sb() # but succeeds in getting the refcnt, # so re-use cgroup root percpu_ref_tryget_live() # alloc sb with cgroup root cgroup_do_mount() cgroup_kill_sb() # alloc another sb with same root cgroup_do_mount() cgroup_kill_sb() We end up using the same cgroup root for two different superblocks, so percpu_ref_kill() will be called twice on the same root when the two superblocks are destroyed. We should fix to make sure the superblock pinning is really successful. Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx> Signed-off-by: Zefan Li <lizefan@xxxxxxxxxx> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- kernel/cgroup.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) --- a/kernel/cgroup.c +++ b/kernel/cgroup.c @@ -1786,7 +1786,7 @@ static struct dentry *cgroup_mount(struc * path is super cold. Let's just sleep a bit and retry. */ pinned_sb = kernfs_pin_sb(root->kf_root, NULL); - if (IS_ERR(pinned_sb) || + if (IS_ERR_OR_NULL(pinned_sb) || !percpu_ref_tryget_live(&root->cgrp.self.refcnt)) { mutex_unlock(&cgroup_mutex); if (!IS_ERR_OR_NULL(pinned_sb)) Patches currently in stable-queue which might be from lizefan@xxxxxxxxxx are queue-3.18/cgroup-avoid-attaching-a-cgroup-root-to-two-different-superblocks.patch