linux-kernel - Re: [PATCH 20/25] sysfs: Rename Support multiple superblocks

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Date:	Wed, 08 Aug 2007 10:35:38 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Tejun Heo <htejun@...il.com>
Cc:	Greg KH <greg@...ah.com>, linux-kernel@...r.kernel.org,
	satyam@...radead.org, cornelia.huck@...ibm.com,
	stern@...land.harvard.edu,
	Linux Containers <containers@...ts.osdl.org>, gregkh@...e.de
Subject: Re: [PATCH 20/25] sysfs: Rename Support multiple superblocks

Tejun Heo <htejun@...il.com> writes:

> Yeah, I think using -ENOENT is better; otherwise, my little head feels
> like exploding.  :-)  

Ok.  I think I know the feeling.

> More importantly, sysfs_get_dentry() seems like it
> would deference ERR_PTR() value.  No?

I'm confused where you are referring to but I will try answer
this.

__sysfs_get_dentry always returns a dentry or NULL if it can't
find the dentry.


I have quoted my final version of sysfs_get_dentry below for discussion.

> struct dentry *sysfs_get_dentry(struct super_block *sb, struct sysfs_dirent *sd)
> {
> 	struct sysfs_dirent *cur;
> 	struct dentry *parent_dentry, *dentry;
> 
> 	/* Bail if this sd won't show up in this superblock */
> 	if (sd->s_parent && sd->s_parent->s_flags & SYSFS_FLAG_TAGGED) {
> 		const void *tag;
> 		tag = sysfs_lookup_tag(sd->s_parent, sb);
> 		if (sd->s_tag.tag != tag)
> 			return NULL;
> 	}

This is the only location where could return NULL the early check
if this operation is possible.

> 	/* Find the first parent which has valid dentry.
> 	 */
> 	dentry = NULL;
> 	cur = sd;
> 	while (!(dentry = __sysfs_get_dentry(sb, cur))) {
> 		if (cur->s_flags & SYSFS_FLAG_REMOVED) {
> 			dentry = ERR_PTR(-ENOENT);
> 			break;
> 		}
> 		cur = cur->s_parent;
> 	}

Here we depend on the fact that sysfs_root is pointed to
by sb->s_root so we know it will always have a dentry.

> 	/* from the found dentry, look up depth times */
> 	while (dentry->d_fsdata != sd) {
> 		/* Find the first ancestor I have not looked up */
> 		cur = sd;
> 		while (cur->s_parent != dentry->d_fsdata)
> 			cur = cur->s_parent;
> 
> 		/* look it up */
> 		parent_dentry = dentry;
> 		dentry = sysfs_add_dentry(parent_dentry, cur);
> 		dput(parent_dentry);
> 
> 		if (IS_ERR(dentry))
> 			break;
> 	}
> 	return dentry;
> }

Eric
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