Re: [RFC] [PATCH 1/5] cgroups: revamp subsys array

[Ben Blum]

On Tue, Dec 08, 2009 at 03:38:43PM +0800, Li Zefan wrote:
> > @@ -1291,6 +1324,7 @@ static int cgroup_get_sb(struct file_system_type *fs_type,
> >  	struct cgroupfs_root *new_root;
> >  
> >  	/* First find the desired set of subsystems */
> > +	down_read(&subsys_mutex);
> 
> Hmm.. this can lead to deadlock. sget() returns success with sb->s_umount
> held, so here we have:
> 
> 	down_read(&subsys_mutex);
> 
> 	down_write(&sb->s_umount);
> 
> On the other hand, sb->s_umount is held before calling kill_sb(),
> so when umounting we have:
> 
> 	down_write(&sb->s_umount);
> 
> 	down_read(&subsys_mutex);
> 
> >  	ret = parse_cgroupfs_options(data, &opts);
> >  	if (ret)
> >  		goto out_err;
> > @@ -1367,6 +1401,8 @@ static int cgroup_get_sb(struct file_system_type *fs_type,
> >  			free_cg_links(&tmp_cg_links);
> >  			goto drop_new_super;
> >  		}
> > +		/* done with subsys stuff and no other failure case */
> > +		up_read(&subsys_mutex);
> >  
> >  		/* EBUSY should be the only error here */
> >  		BUG_ON(ret);
> > @@ -1415,6 +1451,7 @@ static int cgroup_get_sb(struct file_system_type *fs_type,
> >   drop_new_super:
> >  	deactivate_locked_super(sb);
> >   out_err:
> > +	up_read(&subsys_mutex);
> >  	kfree(opts.release_agent);
> >  	kfree(opts.name);

There's another problem here. deactivate_locked_super() calls kill_sb(),
which tries to down_read(&subsys_mutex), and rwsems can't recurse. This
can be fixed by just moving the up_read(&subsys_mutex); at the end to
before each 'goto' in each error case, though, so not a big deal.

-- bblum
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