linux-kernel - Re: [PATCHSET] Sysfs cleanups from Eric W. Biederman

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Message-ID: <46CC4C08.1020408@gmail.com>
Date:	Wed, 22 Aug 2007 23:45:28 +0900
From:	Tejun Heo <htejun@...il.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	cornelia.huck@...ibm.com, greg@...ah.com,
	linux-kernel@...r.kernel.org, satyam@...radead.org,
	stern@...land.harvard.edu, containers@...ts.osdl.org
Subject: Re: [PATCHSET] Sysfs cleanups from Eric W. Biederman

Hello,

Eric W. Biederman wrote:
>> * fix-i_mutex-locking-in-sysfs_get_dentry patch is added at the top
>>   and #14-Don_t-use-lookup_one_len_kern and
>>   #15-vfs-Remove-lookup_one_len_kern are dropped.  This is because #14
>>   contained had a bug where it might created dentry/inode for an
>>   already deleted sysfs_dirent.  I think it's benefitial to keep
>>   single lookup path.
> 
> I think I disagree with the bug spotting.
> 
> At least in net we the sysfs_rename_mutex which keeps parent
> directories from disappearing.  Further we have a reference
> to the leaf sysfs_dirent and are actively manipulating it, so
> the sysfs_dirent should not disappear on us.

sysfs_rename_mutex() keeps out renaming and moving not removing.  Also,
reference prevents sysfs_dirent from being released not from being
removed.  The different in lookup path is that it searches the children
list - sd's are unlinked from children list on removal.

>> * Rewrote simplify-sysfs_get_dentry patch and
>>   #08-Implement-__sysfs_get_dentry,
>>   #09-Move-sysfs_get_dentry-below-__sysfs_get_dentry and
>>   #10-Rewrite-sysfs_get_dentry-in-terms-of-__sysfs_get_dentry are
>>   omitted as __sysfs_get_dentry() isn't used by anyone.
> 
> Right.  __sysfs_get_dentry is an optimization that has makes
> the best case for sysfs_get_dentry O(1) instead of O(depth).
> However this doesn't matter because sysfs_get_dentry is not
> on any fast path and the maximum depth of sysfs directories
> is fairly shallow and programmer controlled.

The reason why sysfs_get_dentry() climbed up first then climbed down was
not because of performance.  It was to support the original shadow
implementation.  Because there was no reliable way to reach a leaf node
from the root, dentries of all shadows are pinned such that they can
serve as the starting point for dentry lookup and the climing up was to
reach that starting point.  Now that the dentry-multiplexing shadow
support is gone, there's no need to do the climing up.

> The only user other user of __sysfs_get_dentry is in the tagged
> directory support, and even that user doesn't strictly need it.
> Although it is a bit silly to populate the dcache just so you
> can invalidate it a moment later...

Yeah, actually the only essential user of that kind of look up is
sysfs_drop_dentry() which is in deletion path and can't fail due to
allocation failure and has the logic open-coded.  I have nothing against
__sysfs_get_dentry().  It was just not needed by the patches I forwarded
this time.  Feel free to include it as you see fit.

I'm currently working on kobj/sysfs separation.  As most internal
implementation is sd based already, the changes are mostly confined to
interface functions but it's still a big change.  I think I'll be able
to post the patches in this week or early next week at the latest.

Thanks.

-- 
tejun
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