RCU is not safe enough to protect __d_lookup ?

Hi, all: I am studying the dcache in VFS recently . I've found that hlist of dcache is already protected by RCU in __d_lookup.Is it necessary for the function -- d_lookup using sequence lock to protect the hlist again? struct dentry * d_lookup(struct dentry * parent, struct qstr * name){ struct dentry * dentry = NULL; unsigned long seq; do { seq = read_seqbegin(&rename_lock); dentry = __d_lookup(parent, name); if (dentry) break; } while (read_seqretry(&rename_lock, seq)); return dentry;} Thanks in advance--------------------------------Rock Lee