Messages in this thread | | | Date | Wed, 12 Jun 2013 17:38:13 -0700 | Subject | Re: [PATCH RFC ticketlock] Auto-queued ticketlock | From | Linus Torvalds <> |
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On Wed, Jun 12, 2013 at 5:20 PM, Al Viro <viro@zeniv.linux.org.uk> 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.
Linus
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