学步园

      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

static int 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

static struct 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;
}