linux-kernel - Re: [PATCH RFC ticketlock] v2 Auto-queued ticketlock

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Message-ID: <20130611174951.GQ5146@linux.vnet.ibm.com>
Date:	Tue, 11 Jun 2013 10:49:51 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	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>,
	Steven Rostedt <rostedt@...dmis.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>,
	Michel Lespinasse <walken@...gle.com>,
	Waiman Long <waiman.long@...com>
Subject: Re: [PATCH RFC ticketlock] v2 Auto-queued ticketlock

On Tue, Jun 11, 2013 at 10:35:53AM -0700, Linus Torvalds wrote:
> Hmm. Something just struck me when reading this patch..
> 
> Our memory ordering semantics in our *current* locks are very very
> subtle. We have just a "barrier()" between the
> 
>     inc.head = ACCESS_ONCE(lock->tickets.head);
>     if (inc.head == inc.tail)
>         break; /* success */
> 
> and the inside of the locked region.
> 
> I think it's safe because of the new memory ordering semantics (loads
> are in-order, and stores only move *down*), but there's not even a
> comment about it.
> 
> So let's at least comment the current locks before making them even
> more complex and subtle..

Would it make sense to have something like an smp_tso() that was a
compiler barrier for TSO systems (x86, s390, sparc, etc.) but that
emitted the needed memory-barrier instruction for weakly ordered systems?

Seems to me to be easy to do, and helps describe the intent better.

							Thanx, Paul

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