Re: [PATCH RFC ticketlock] Auto-queued ticketlock

From: Linus Torvalds
Date: Wed Jun 12 2013 - 20:38:38 EST

On Wed, Jun 12, 2013 at 5:20 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> Actually, dget_parent() change might be broken; the thing is, the assumptions
> are more subtle than "zero -> non-zero only happens under ->d_lock". It's
> actually "new references are grabbed by somebody who's either already holding
> one on the same dentry _or_ holding ->d_lock". That's what d_invalidate()
> check for ->d_count needs for correctness - caller holds one reference, so
> comparing ->d_count with 2 under ->d_lock means checking that there's no other
> holders _and_ there won't be any new ones appearing.

For the particular case of dget_parent() maybe dget_parent() should
just double-check the original dentry->d_parent pointer after getting
the refcount on it (and if the parent has changed, drop the refcount
again and go to the locked version). That might be a good idea anyway,
and should fix the possible race (which would be with another cpu
having to first rename the child to some other parent, and the
d_invalidate() the original parent)

That said, the case we'd really want to fix isn't dget_parent(), but
just the normal RCU lookup finishing touches (the__d_rcu_to_refcount()
case you already mentioned) . *If* we could do that without ever
taking the d_lock on the target, that would be lovely. But it would
seem to have the exact same issue. Although maybe the
dentry_rcuwalk_barrier() thing ends up solving it (ie if we had a
lookup at a bad time, we know it will fail the sequence count test, so
we're ok).

Subtle, subtle.

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