Ocfs2 devel - Sep 2014 - One question about the usage of dlm_domain_lock of OCFS2

As I reviewing the code, there are some of usages I does not understand.
extern spinlock_t dlm_domain_lock;
extern struct list_head dlm_domains;

I think dlm_domain_lock is used to control the access of dlm_domains.

Why not prefer dlm_lock to dlm_domain_lock in the function
dlm_try_to_join_domain?
        spin_lock(&dlm_domain_lock);
        dlm->dlm_state = DLM_CTXT_JOINED;
        dlm->num_joins++;
        spin_unlock(&dlm_domain_lock);

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