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SubjectRe: [PATCH] configfs: Silence lockdep on mkdir(), rmdir() and configfs_depend_item()
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On Thu, 2008-12-18 at 14:58 -0800, Joel Becker wrote:
> On Thu, Dec 18, 2008 at 01:28:28PM +0100, Peter Zijlstra wrote:
> > In fact, both (configfs) mkdir and rmdir seem to synchronize on
> > su_mutex..
> >
> > mkdir B/C/bar
> >
> > C.i_mutex
> > su_mutex
> >
> > vs
> >
> > rmdir foo
> >
> > parent(foo).i_mutex
> > foo.i_mutex
> > su_mutex
> >
> >
> > once holding the rmdir su_mutex you can check foo's user-content, since
> > any mkdir will be blocked. All you have to do is then re-validate in
> > mkdir's su_mutex that !IS_DEADDIR(C).
>
> We explicitly do not take any i_mutex locks after taking
> su_mutex. That's an ABBA risk. su_mutex protects the hierarchy of
> config_items. i_mutex protects the vfs view thereof.

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.

> If you look in mkdir, we take su_mutex, get a new item from the
> client subsystem, then drop su_mutex.

All you need to do before dropping su_mutex again is checking
IS_DEADDIR(), if so, you just fail the whole mkdir() no extra i_mutex's
needed.

> After that, we go about building
> our filesystem structure, using i_mutex where appropriate.

Sure, but its ok to grow the default groups non-atomically, right? mkdir
will only need to check that everything is empty in as far as it has
been linked, and ensure the not yet linked entries won't be.

> More
> importantly is rmdir(2), where we use i_mutex in
> configfs_detach_group(), but are not holding su_sem. Only when
> configfs_detach_group() has successfully returned and we have torn down
> the filesystem structure do we take su_mutex and tear down the
> config_item structure.

The only thing that matters is that you can hold su_mutex inside
i_mutex.


configfs_rmdir( "foo" )
{
/* we hold i_mutex for foo and its parent */

mutex_lock(&subsys->su_mutex);
if (default_tree_empty())
mark_default_tree_dead();
else
ret = -EBUSY;
mutex_unlock(&subsys->su_mutex);

if (ret)
return ret;

/* do actual unlink foo */
}


configfs_mkdir( "B/A/bar" )
{
/* we hold i_mutex for A */

mutex_lock(&subsys->su_mutex);
if (IS_DEADDIR(A))
ret = -EINVAL; /* or whatever */

/* increase A's use count, so default_tree_empty() will fail. *
inc_A_or_subsys_use_count();
mutex_unlock(&subsys->su_mutex);
if (ret)
return ret;

/* do actual mkdir */
}


Surely something along these lines ought to work?



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