linux-kernel - Re: [PATCH RFC ticketlock] Auto-queued ticketlock

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Date:	Wed, 12 Jun 2013 17:38:13 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Davidlohr Bueso <davidlohr.bueso@...com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	"?????????" <laijs@...fujitsu.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Josh Triplett <josh@...htriplett.org>, niv@...ibm.com,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	David Howells <dhowells@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Darren Hart <darren@...art.com>,
	"Fr??d??ric Weisbecker" <fweisbec@...il.com>,
	Silas Boyd-Wickizer <sbw@....edu>,
	Waiman Long <Waiman.Long@...com>
Subject: Re: [PATCH RFC ticketlock] Auto-queued ticketlock

On Wed, Jun 12, 2013 at 5:20 PM, Al Viro <viro@...iv.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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