Re: [PATCH] configfs: Silence lockdep on mkdir(), rmdir() andconfigfs_depend_item()

[Joel Becker]

On Mon, Jan 26, 2009 at 01:30:09PM +0100, Peter Zijlstra wrote:
> I don't think I was suggesting that. All you need is to serialize any
> mkdir/creat against the rmdir of the youngest non-default group, and you
> can do that by holding su_mutex.
> 
> In rmdir, you already own all the i_mutex instances you need to uncouple
> the whole tree, all you need to do is validate that its indeed empty --
> you don't need i_mutex's for that, because you're holding su_mutex, and
> any concurrent mkdir/creat will be blocking on that.
> 
> If you find it empty, just mark everybody DEAD, drop su_mutex and
> decouple. All concurrent mkdir/creat thingies that were blocking will
> now bail because their parent is found DEAD.

	Right, that's what I was talking about when I said:

> > 	Now look in detach_groups().  We drop the groups children before
> > marking them DEAD.  Louis' plan, I think, is to perhaps mark a group
> > DEAD, disconnect it from the vfs, and then operate on its children.  In
> > this fashion, perhaps we can unlock the trailing lock like a normal VFS
> > operation.
> > 	This will require some serious auditing, however, because now
> > vfs functions can get into the vfs objects behind us.  And more vfs
> > changes affect us.  Whereas the current locking relies on the vfs's
> > parent->child lock ordering only, something that isn't likely to be
> > changed.
 
	That is, the vfs has already walked past this directory,
dropped i_mutex, and is in a child default group holding its i_mutex.
It wants to mkdir(2) down there.  You're saying that, if mkdir(2) holds
su_mutex higher up, it can check S_DEAD and compare with us, and that's
exactly the scheme I mentioned in the first of the quoted paragraphs
(Louis proposed it a while back).  Thus, though we don't hold i_mutex on that
child, it will eventually either a) have gotten su_mutex first, and will
cause rmdir to ENOTEMPTY or b) have gotten su_mutex second, will see
S_DEAD, and return -ENOENT.  As far as preventing mkdir(2), I don't see
why it wouldn't work.
	The issue is in that second quoted paragraph.  We know that the
VFS can look up our children if we're not holding i_mutex.  In fact,
cached_lookup() can find them without i_mutex.  Now, we know that
mkdir(2) and rmdir(2) will block at su_mutex.  But what about all other
file operations, both on the child directories *and* the attribute
files?  For attribute files, we prevent access at creation time with a
flag.  We can trust the flag because we hold i_mutex.  This might hold
anyway because we're holding that toplevel i_mutex.  At teardown time,
though, we know they can't be found because of i_mutex.  Now we don't
have that protection for processes that are farther down the tree.
	But the bigger issue is just the plain regular operations on our
directories.  An example is ->readdir().  We currently lock out
->readdir() during rmdir(2) with our holding of i_mutex.  However, if we
are not holding i_mutex, vfs_readdir() can call into our ->readdir()
right as we're tearing things down.  We may not have gotten to S_DEAD
yet in the rmdir(2) path, and the two will race.  We can't take su_mutex
in ->readdir(), because that sort of solution effectively says "we have
to take su_mutex for all operations", and we end up serializing all
operations on a subsustem.
	Now, I can think of a way to make ->readdir() safe without
su_mutex.  But what other operation is next?  How do I know I have them
all?  How do I notice when someone adds a new operation or code path to
the generic code that I have to protect against?  With i_mutex, I *know*
that everyone has agreed that is the gatekeeper.  Without it...
	*That's* the big worry.  That's what I'm worried about being
sure of.  I'd love to hear a solution that we know will work, or at
least move towards one.

Joel
	
PS:  And we haven't even talked about configfs_depend_item() yet.

-- 

Life's Little Instruction Book #252

	"Take good care of those you love."

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