linux-kernel - Re: [PATCH 1/3][BUGFIX] configfs: Introduce configfs_dirent

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Message-ID: <20080613024130.GD20581@mail.oracle.com>
Date:	Thu, 12 Jun 2008 19:41:31 -0700
From:	Joel Becker <Joel.Becker@...cle.com>
To:	Louis Rilling <Louis.Rilling@...labs.com>
Cc:	linux-kernel@...r.kernel.org, ocfs2-devel@....oracle.com
Subject: Re: [PATCH 1/3][BUGFIX] configfs: Introduce configfs_dirent_lock

On Fri, Jun 13, 2008 at 12:25:58AM +0200, Louis Rilling wrote:
> On Thu, Jun 12, 2008 at 12:13:48PM -0700, Joel Becker wrote:
> > On Thu, Jun 12, 2008 at 03:31:27PM +0200, Louis Rilling wrote:
> > > Locking rules for configfs_dirent linkage mutations are the same plus the
> > > requirement of taking configfs_dirent_lock. For configfs_dirent walking, one can
> > > either take appropriate i_mutex as before, or take configfs_dirent_lock.
> > 
> > 	Nope, you *must* take configfs_dirent_lock now.  You've removed
> > i_mutex protection in the last patch.
> 
> Oh well. Do you mean because of CONFIGFS_USET_DROPPING being set without
> i_mutex locked? This is the only mutation (except in the s_links patch) done
> without i_mutex locked. I thought that actually either other
> configfs_dirent traversals like readdir() and dir_lseek() would prevent
> detach_prep() from succeeding because they add dirents before, or are done in
> places where detach_prep() cannot do harm because new_dirent() fails whenever it
> sees CONFIGFS_USET_DROPPING: detach_attrs() and detach_groups()
> must ignore CONFIGFS_USET_DROPPING, depend_prep() is protected since the
> whole path is locked from configfs root, lookup() can succeed since at worst its
> result will be invalidated when actually detaching the default groups. The only
> function for which I can not figure out is configfs_hash_and_remove(), but it is
> not used.

	I don't mean that your code is wrong, I mean that the comment is
unclear.  The locking rules aren't "you can use i_mutex or dirent_lock,
take your pick".  I think you are right that configfs_detach_prep() is
safe to set dropping as it does without i_mutex.
	This is related to the discussion below about VFS visible
changes (i_mutex protection) vs subsystem internal changes (dirent_lock
protection).  The protections have different scope, but your comment
made them sound interchangable. 

> 	I admit that the case of symlink() needs an extra check to ensure
> that the target is not about to be removed. The bug was already there
> though, right?
> 	Anyway, if it looks conceptually simpler to use
> configfs_dirent_lock (probably better a mutex in that case) wherever
> i_mutex are supposed to protect configfs_dirent traversals, I'm ok with it.

	Leave it as a spinlock.
	Going over the changes, I was pretty convinced your detach_prep
was safe vis-a-vis mkdir.  You're under i_mutex for the immediate
directory, and both attach_* and detach_* are under the immediate
i_mutex when they make the change.  Also, you have your readdir and
lookup walking s_children without a lock.  I *think* that's safe, because
it's also against the immediate directory, and thus the vfs is holding
i_mutex for you.
 
> And we should not take other i_mutex in populate_groups() and
> populate_attrs(), otherwise deadlocks could happen.

	Huh, we certainly should.  perhaps you are speaking as if we
were turning dirent_lock into a mutex.  We're not turning dirent_lock
into a mutex yet.

> > 	Now, the only thing that sees this intermediate condition is
> > configfs itself.  Everyone else is protected by i_mutex.  I guess it's
> > OK - but can you comment that fact?  i_mutex does *not* protect
> > traversal of the configfs_dirent tree, but it does prevent the outside
> > world from seeing the intermediate states.
> 
> The only intermediate conditions that may hurt one's mind is that an
> mkdir() (resp. symlink()) racing with an rmdir() can successfuly call
> make_item()/make_group() (resp. allow_link()) and immediately fail when
> finalizing with attach_item()/attach_group() (resp. create_link()). So, from
> userspace and the VFS this seems like "mkdir foo/bar/baz" simply failed because
> of "rmdir foo", while at the same time from the subsystem point of view this
> seems like userspace did "mkdir foo/bar/baz; rmdir foo/bar/baz; rmdir foo".
> As I pointed out in the rename fix, this however can already happen when
> attach_item()/attach_group() (resp. create_link()) fails because of
> memory pressure for instance.

	I'm not even sure what you said here :-)

Joel

-- 

"Egotist: a person more interested in himself than in me."
         - Ambrose Bierce 

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