1. /*
   
2.  * It's more convoluted than I'd like it to be, but... it's still fairly
   
3.  * small and for now I'd prefer to have fast path as straight as possible.
   
4.  * It _is_ time-critical.
   
5.  */
   
6. static int do_lookup(struct nameidata *nd, struct qstr *name,
   
7.          struct path *path)
   
8. {
   
9.     struct vfsmount *mnt = nd->path.mnt;
   
10.     struct dentry *dentry, *parent;
    
11.     struct inode *dir;
    
12.     /*
    
13.      * See if the low-level filesystem might want
    
14.      * to use its own hash..
    
15.      */
16.     /* nd->path.dentry代表的是parent dentry，如果父节点代表的文件系统有自己的hash函数，在这里需要更新qstr *name里面的hash值 
17.         比如说要查找"/mnt/tmp1"里面的tmp1分量,那么nd->path.dentry就是"/mnt"的dentry
    
18.     */
    
19.     if (nd->path.dentry->d_op && nd->path.dentry->d_op->d_hash) {
    
20.         int err = nd->path.dentry->d_op->d_hash(nd->path.dentry, name);
    
21.         if (err < 0)
    
22.             return err;
    
23.     }
    

    /* 先调用__d_lookup在hash表里查找。这里的查找只是在内存中查找，相当于在cache里面查找。 */

1.     dentry = __d_lookup(nd->path.dentry, name);
   
2.     if (!dentry)/* 如果内存里查找不到，需要goto need_lookup处，执行文件系统的查找 */
   
3.         goto need_lookup;
   
4.     if (dentry->d_op && dentry->d_op->d_revalidate)
   
5.         goto need_revalidate;
   
6. done:
7.     /* 这里代表查找成功，更新path
8.        比如说要查找"/mnt/tmp1"里面的"tmp1"分量, 到这里就找到了"tmp1"这个分量的dentry和mnt结构
9.     */
   
10.     path->mnt = mnt;
    
11.     path->dentry = dentry;
12.     /* 假如/mnt/tmp1是一个mount point，比如 执行过"mount /dev/sdc1 /mnt/tmp1 */
    
13.     __follow_mount(path);
    
14.     return 0;
    
15. 
    
16. need_lookup:
17.     /* 比如说要查找"/mnt/tmp1"里面的tmp1分量, __d_lookup没有查找成功，这里的parent是"/mnt"对应的dentry
18.         dir是/mnt对应的inode
19.     */
    
20.     parent = nd->path.dentry;
    
21.     dir = parent->d_inode;
    
22. 
    
23.     mutex_lock(&dir->i_mutex);
    
24.     /*
    
25.      * First re-do the cached lookup just in case it was created
    
26.      * while we waited for the directory semaphore..
    
27.      *
    
28.      * This could use version numbering or similar to
    
29.      * avoid unnecessary cache lookups.
    
30.      *
    
31.      * The "dcache_lock" is purely to protect the RCU list walker
    
32.      * from concurrent renames at this point (we mustn't get false
    
33.      * negatives from the RCU list walk here, unlike the optimistic
    
34.      * fast walk).
    
35.      *
    
36.      * so doing d_lookup() (with seqlock), instead of lockfree __d_lookup
    
37.      */
    
38.     dentry = d_lookup(parent, name);
    
39.     if (!dentry) {
    
40.         struct dentry *new;
    
41. 
    
42.         /* Don't create child dentry for a dead directory. */
    
43.         dentry = ERR_PTR(-ENOENT);
    
44.         if (IS_DEADDIR(dir))
    
45.             goto out_unlock;
    
46.        
    
47.         /* 分配一个dentry结构 */
    
48.         new = d_alloc(parent, name);
    
49.         dentry = ERR_PTR(-ENOMEM);
    
50.         if (new) {
51.             /* 这里才是到文件系统里面查找
52.                比如说proc fs，对应的lookup函数是proc_root_lookup，proc_root_lookup执行成功后会返回NULL，失败返回一个非NULL值 */
    
53.             dentry = dir->i_op->lookup(dir, new, nd);
    
54.             if (dentry)    /* 执行失败，最终会把new释放掉 */
    
55.                 dput(new);
    
56.             else /* 执行成功 */
    
57.                 dentry = new;
    
58.         }
    
59. out_unlock:
    
60.         mutex_unlock(&dir->i_mutex);
    
61.         if (IS_ERR(dentry))
    
62.             goto fail;
    
63.         goto done;
    
64.     }
    
65. 
    
66.     /*
    
67.      * Nasty case: the cache was re-populated while
    
68.      * we waited on the semaphore. Need to revalidate.
    
69.      */
    
70.     mutex_unlock(&dir->i_mutex);
    
71.     if (dentry->d_op && dentry->d_op->d_revalidate) {
    
72.         dentry = do_revalidate(dentry, nd);
    
73.         if (!dentry)
    
74.             dentry = ERR_PTR(-ENOENT);
    
75.     }
    
76.     if (IS_ERR(dentry))
    
77.         goto fail;
    
78.     goto done;
    
79. 
    
80. need_revalidate:
    
81.     dentry = do_revalidate(dentry, nd);
    
82.     if (!dentry)
    
83.         goto need_lookup;
    
84.     if (IS_ERR(dentry))
    
85.         goto fail;
    
86.     goto done;
    
87. 
    
88. fail:
    
89.     return PTR_ERR(dentry);
    
90. }