linux-kernel - Re: [PATCH 2/2] configfs: Rework configfs_depend

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Date:	Tue, 27 Jan 2009 20:13:53 -0800
From:	Joel Becker <Joel.Becker@...cle.com>
To:	Louis Rilling <louis.rilling@...labs.com>
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	cluster-devel@...hat.com, swhiteho@...hat.com, peterz@...radead.org
Subject: Re: [PATCH 2/2] configfs: Rework configfs_depend_item() locking
	and make lockdep happy

On Thu, Dec 18, 2008 at 07:00:18PM +0100, Louis Rilling wrote:
> configfs_depend_item() recursively locks all inodes mutex from configfs root to
> the target item, which makes lockdep unhappy. The purpose of this recursive
> locking is to ensure that the item tree can be safely parsed and that the target
> item, if found, is not about to leave.
> 
> This patch reworks configfs_depend_item() locking using configfs_dirent_lock.
> Since configfs_dirent_lock protects all changes to the configfs_dirent tree, and
> protects tagging of items to be removed, this lock can be used instead of the
> inodes mutex lock chain.
> This needs that the check for dependents be done atomically with
> CONFIGFS_USET_DROPPING tagging.
> 
> Now lockdep looks happy with configfs.

	This looks almost, but not quite right.
	In the create path, we do configfs_new_dirent() before we set
sd->s_type.  But configfs_new_dirent() attaches sd->s_sibling.  So, in
aonther thread, configfs_depend_prep() can traverse this s_sibling
without CONFIGFS_USET_CREATING being set.  This turns out to be safe
because CONFIGFS_DIR is also not set - but boy I'd like a comment about
that.
	What if we're in mkdir(2) in one thread and another thread is  
trying to pin the parent directory?  That is, we are inside
configfs_mkdir(parent, new_dentry, mode).  The other thread is doing
configfs_depend_item(subsys, parent).  With this patch, the other thread
will not take parent->i_mutex.  It will happily determine that
parent is part of the tree and bump its s_dependent with no locking.  Is
this OK?
	If it is - isn't this patch good without any other reason?  That
is, aside from the issues of lockdep, isn't it better for
configfs_depend_item() to never have to worry about the VFS locks other
than the configfs root?

Joel


-- 

 The zen have a saying:
 "When you learn how to listen, ANYONE can be your teacher."

Joel Becker
Principal Software Developer
Oracle
E-mail: joel.becker@...cle.com
Phone: (650) 506-8127
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