对于一个文件路径的分量，如果其不为'.'和'..'则属于普通文件名，普通文件名的解析由do_lookup()函数来处理
staticint do_lookup(struct nameidata *nd, struct qstr *name,
struct path *path)
{
struct vfsmount *mnt = nd->path.mnt;
struct dentry *dentry = __d_lookup(nd->path.dentry, name);//查找name对应的dentry
if (!dentry)//dentry不存在，跳转至need_lookup
goto need_lookup;
/*如果底层文件系统中定义了d_revalidate函数，则要判断目录项是否有效，以保证一致性，
该函数是针对于网络文件系统存在的*/
if (dentry->d_op && dentry->d_op->d_revalidate)
goto need_revalidate;
done:
path->mnt = mnt;
path->dentry = dentry;
/*这里由于path往下走了一层，因此要调用__follow_mount()判断dentry对应的目录下是否挂载了其他的文件系统，
以保证对应的mnt是正确的*/
__follow_mount(path);
return 0;
need_lookup:
/*没有找到name对应的dentry,则要创建新的dentry并从磁盘中读取数据保存在dentry中*/
dentry = real_lookup(nd->path.dentry, name, nd);
if (IS_ERR(dentry))
goto fail;
goto done;
need_revalidate:
dentry = do_revalidate(dentry, nd);
if (!dentry)
goto need_lookup;
if (IS_ERR(dentry))
goto fail;
goto done;
fail:
return PTR_ERR(dentry);
}
可以想象，搜索一个文件(目录)时，首先肯定要在dentry缓存中查找，当缓存中查找不到对应的dentry时，才需要从磁盘中查找，并新建一个dentry，将磁盘中的数据保存到其中。找到了目标dentry后，就将相应的信息保存到path中，这里因为路径向下进了一层，因此要判断下层目录是否有新的文件系统挂载的问题，和上文讨论的类似，因此要通过__follow_mount()函数判断是否有文件系统挂载在该目录下，另外，对于网络文件系统，还要通过文件系统中定义的d_revalidate()函数来判断该dentry是否有效以保证一致性。
先来看看在dentry缓存中查找的过程
struct dentry * __d_lookup(struct dentry * parent, struct qstr * name)
{
unsigned int len = name->len;
unsigned int hash = name->hash;
const unsigned char *str = name->name;
struct hlist_head *head = d_hash(parent,hash);//通过parent的地址和hash(hash是name的哈希值)进行定位
struct dentry *found = NULL;
struct hlist_node *node;
struct dentry *dentry;
rcu_read_lock();
hlist_for_each_entry_rcu(dentry, node, head, d_hash) {//扫描head对应的碰撞溢出表
struct qstr *qstr;
if (dentry->d_name.hash != hash)//name的hash值不相符，则放弃该dentry
continue;
if (dentry->d_parent != parent)//父目录不一样，则放弃该dentry
continue;
spin_lock(&dentry->d_lock);
/*
* Recheck the dentry after taking the lock - d_move may have
* changed things. Don't bother checking the hash because we're
* about to compare the whole name anyway.
*/
if (dentry->d_parent != parent)
goto next;
/* non-existing due to RCU? */
if (d_unhashed(dentry))
goto next;
/*
* It is safe to compare names since d_move() cannot
* change the qstr (protected by d_lock).
*/
/*当确保了父目录和文件名的哈希值与目标dentry的一致性后，接下来就只用匹配文件名了*/
qstr = &dentry->d_name;//取当前dentry的文件名
/*如果父目录文件系统定义了比较文件名的方法，则调用之*/
if (parent->d_op && parent->d_op->d_compare) {
if (parent->d_op->d_compare(parent, qstr, name))
goto next;
} else {//如果没定义
if (qstr->len != len)//先确定长度是否相等
goto next;
if (memcmp(qstr->name, str, len))//再比较内存
goto next;
}
atomic_inc(&dentry->d_count);
found = dentry; //这里表明找到了目标dentry
spin_unlock(&dentry->d_lock);
break;
next:
spin_unlock(&dentry->d_lock);
}
rcu_read_unlock();
return found;
}
d_hash()函数将父目录dentry的地址和所要查找的文件名的哈希值组合起来，重新构建一个哈希值，并根据其定位到
dentry_hashtable哈希表中，dentry_hashtable是dentry缓存的一部分，所有的dentry都会保存在dentry_hashtable中，这样一来，就得到了一个哈希表的溢出链表的表头，即代码中的head变量。下面的工作就是扫描这个链表，并从中查找真正的目标。
如果在dentry_hashtable中没能找到目标dentry，则通过real_lookup()函数从磁盘中查找
staticstruct dentry * real_lookup(struct dentry * parent, struct qstr * name, struct nameidata *nd)
{
struct dentry * result;
struct inode *dir = parent->d_inode;
mutex_lock(&dir->i_mutex);
/*
* First re-do the cached lookup just in case it was created
* while we waited for the directory semaphore..
*
* FIXME! This could use version numbering or similar to
* avoid unnecessary cache lookups.
*
* The "dcache_lock" is purely to protect the RCU list walker
* from concurrent renames at this point (we mustn't get false
* negatives from the RCU list walk here, unlike the optimistic
* fast walk).
*
* so doing d_lookup() (with seqlock), instead of lockfree __d_lookup
*/
//再从磁盘读取前再进行一次d_lookup尝试，因为之前等待互斥锁时有可能已经创建了相应的dentry
result = d_lookup(parent, name);
/*下面进行dentry的创建*/
if (!result) {
struct dentry *dentry;
/* Don't create child dentry for a dead directory. */
result = ERR_PTR(-ENOENT);
if (IS_DEADDIR(dir))
goto out_unlock;
/*分配一个dentry并进行相应的初始化*/
dentry = d_alloc(parent, name);
result = ERR_PTR(-ENOMEM);
if (dentry) {
/*调用特定于文件系统的lookup函数从磁盘中读取数据并将dentry添入散列表*/
result = dir->i_op->lookup(dir, dentry, nd);
if (result)
dput(dentry);
else
result = dentry;
}
out_unlock:
mutex_unlock(&dir->i_mutex);
return result;
}
/*
* Uhhuh! Nasty case: the cache was re-populated while
* we waited on the semaphore. Need to revalidate.
*/
mutex_unlock(&dir->i_mutex);
if (result->d_op && result->d_op->d_revalidate) {
result = do_revalidate(result, nd);
if (!result)
result = ERR_PTR(-ENOENT);
}
return result;
}