Messages in this thread | | | Date | Wed, 25 Jun 2014 11:00:53 -0400 | From | Tejun Heo <> | Subject | Re: [PATCH 5/5] cgroup: fix a race between cgroup_mount() and cgroup_kill_sb() |
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Hey,
On Wed, Jun 25, 2014 at 09:56:31AM +0800, Li Zefan wrote: > > Hmmm? Why does that matter? The only region in cgroup_mount() which > > needs to be put inside such mutex would be root lookup, no? > > unfortunately that won't help. I think what you suggest is: > > cgroup_mount() > { > mutex_lock(); > lookup_cgroup_root(); > mutex_unlock(); > kernfs_mount(); > } > > cgroup_kill_sb() > { > mutex_lock(); > percpu_ref_kill(); > mutex_Unlock(); > kernfs_kill_sb(); > } > > See, we may still be destroying the superblock after we've succeeded > in getting the refcnt of cgroup root.
Sure, but now the decision to kill is synchronized so the other side can interlock with it. e.g.
cgroup_mount() { mutex_lock(); lookup_cgroup_root(); if (root isn't killed yet) root->this_better_stay_alive++; mutex_unlock(); kernfs_mount(); }
cgroup_kill_sb() { mutex_lock(); if (check whether root can be killed) percpu_ref_kill(); mutex_unlock(); if (the above condition was true) kernfs_kill_sb(); }
-- tejun
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