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

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Date:	Thu, 13 Jun 2013 01:20:58 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
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 05:01:19PM -0700, Linus Torvalds wrote:
> I'd actually suggest we do *not* remove any existing d_lock usage
> outside of the particular special cases we want to optimize, which at
> least from Davidlohr's profile is just dput() (which has shown up a
> lot before) and dget_parent() (which I'm not sure why it happens so
> much on his load, but it really seems trivially safe to optimistically
> do under just the RCU lock).

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.

Consider the following situation:
X is dentry of a/b
Y is dentry of a/b/c
Z is dentry of d/e

A holds a reference to Y and enters dget_parent(Y)
B holds a reference to X and enters d_invalidate(X)
A picks the value of Y->d_parent (== X)
C moves Y to Z
B grabs ->d_lock on X
B checks X->d_count; it's 1, we deduce that no other references exist or
are going to appear
A does atomic_inc_not_zero(&X->d_count).  And since it's not zero (it's 1,
actually), we've just grabbed an extra reference on X that was not going
to appear according to B...

> That said, I do wonder if we could do something like
> "atomic_inc_not_zero()" on the d_count, and only if it is zero (which
> won't be horribly unusual, since for leaf dentries that nobody else is
> using) we'd do the whole locking sequence.

Same correctness issue as above, I'm afraid...

> End result: I think it would be interesting to try this all out, and
> it could be a noticeable win under some cases, but it *definitely*
> needs a lot of timing and testing to see which ways it goes..

*nod*

What's more, we need the underlying assumptions documented very clearly for
any such change; it's _not_ as simple as "protect transitions from zero to
non-zero and we are done" ;-/
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