linux-kernel - Re: [PATCH 04/20] sysfs: Handle the general case of removing of directories with subdirectories

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Date:	Thu, 21 May 2009 16:36:10 +0900
From:	Tejun Heo <tj@...nel.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	linux-kernel@...r.kernel.org,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	linux-fsdevel@...r.kernel.org,
	"Eric W. Biederman" <ebiederm@...stanetworks.com>
Subject: Re: [PATCH 04/20] sysfs: Handle the general case of removing of directories
 with subdirectories

Eric W. Biederman wrote:
>> I agree we should be heading this way but what happens to attributes
>> or directories living below the subdirectories?  If it's gonna handle
>> recursive case, I think it better do it properly.  I had patches of
>> similar effect.
> 
> I do handle it properly.  sysfs_get_one finds the deepest child of the
> first directory entry.  Then I remove it.  And I repeat until done.
> 
> The locking is correct, something that is much more difficult to
> tell with your version.

Why? :-)

> By grabbing and dropping the sysfs_mutex things are simpler, and they
> get even simpler in future patches.
> 
> Now looking at that code in detail there is a question of what happens if
> we add a directory entry while we are recursively deleting a directory.
> Neither your patch, my patch, nor the existing code handle that case
> (assuming the sysfs_dirent) was looked up before it is removed from it's
> parent directory.  I expect another patch is called for to plug that
> theoretical gap.  
> 
> I expect the way to close that hole is to have an extra flag that says
> we are removing a directory entry and refuse to add if that flag is
> set.
> 
> I would prefer to only remove empty directories.  But when I
> instrumented things up I found cases where that does indeed happen.

IIRC, my version did the whole thing while holding sysfs_mutex, so
it's safe against such races.  I can't really see why ops like this
can't be atomic in sysfs.  I don't really care how things are done but
please make it atomic.

Thanks.

-- 
tejun
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