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

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Message-ID: <CA+55aFzJoxGH5J0yuBnWHcpxodymrKbySJ7v=vnfn9TSk7H5eA@mail.gmail.com>
Date:	Wed, 12 Jun 2013 13:37:12 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Davidlohr Bueso <davidlohr.bueso@...com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Waiman Long <Waiman.Long@...com>
Cc:	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>,
	赖江山 <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>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Silas Boyd-Wickizer <sbw@....edu>
Subject: Re: [PATCH RFC ticketlock] Auto-queued ticketlock

On Wed, Jun 12, 2013 at 1:03 PM, Davidlohr Bueso <davidlohr.bueso@...com> wrote:
>
> Waiman's dcache patchet were actually an attempt to address these exact
> issues: http://lkml.org/lkml/2013/5/22/716

Ok, looking at that patch-set, I think it has the same race with not
atomically getting the d_lock spinlock and d_count going down to zero
in dput(). And Waiman clearly didn't know about
"atomic_inc_not_zero()" or "atomic_dec_and_lock()" that are designed
for exactly the "increment if already nonzero" and "decrement without
taking the lock if we're not going down to zero" cases.

As outlined, I'm also not at all sure that the whole seqrw-lock thing
that Waiman did is really necessary - I think the optimistic
dget_parent() might be sufficient.

                 Linus
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