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SubjectRe: [PATCH 1/2] vfs: Do not allow mnt_longterm to go negative
On Tue, May 29, 2012 at 12:56:44PM +0200, Luk?? Czerner wrote:
> Anyone can take a look at this ? It's pretty pressing I would say
> since this bug will not only disallow to unattach the loop devices
> but also make it unable to actually unmount the file system!

I agree that it's a bug, but I don't believe that this is a proper
fix. We can do it much simpler. Look: making these guys longterm
happens exactly at the same points where we set their ->mnt_ns non-NULL.
Remember that "longterm" == "internal kernel mount, mount in some
namespace, someone's root or pwd". So the problem is that umount_tree()
is that umount_tree() blindly assumes that it's killing a longterm
ref, even though it may be called when dissolving a tree that never
made it into anyone's namespace. Fine, so let's just have that
sucker do
if (p->mnt_ns)
p->mnt_ns = NULL;
instead and that's all it takes. Alternatively, we can just turn the
code in mntput_no_expire() into
if (likely(mnt->mnt_ns || atomic_read(...))
and get rid of longterm/shortterm logics around ->mnt_ns completely.
Short-term, pardon the bad pun, I'm going with the first variant; it's
less intrusive. Longer term I think we want to get rid of playing with
mnt_longterm in there and just check ->mnt_ns as well when deciding to
go for the fast path in mntput_no_expire().

Fixed in my tree, will push in a couple of hours. And that'll need to
go into -stable as well. Thanks for catching that sucker...

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