linux-kernel - Re: futex: clarification needed with drop_futex_key

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Message-ID: <20160331073642.GE3408@twins.programming.kicks-ass.net>
Date:	Thu, 31 Mar 2016 09:36:42 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Petros Koutoupis <petros@...roskoutoupis.com>
Cc:	Mike Galbraith <umgwanakikbuti@...il.com>,
	linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Davidlohr Bueso <dave@...olabs.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	catalin.marinas@....com, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: futex: clarification needed with drop_futex_key_refs and memory
 barriers

On Wed, Mar 30, 2016 at 08:45:10PM -0500, Petros Koutoupis wrote:
> > But this is not a correctness (nor ordering) issue; but purely an
> > architectural side-effect. Furthermore; some proposed changes:
> > 
> >   http://marc.info/?l=linux-kernel&m=145400059704564&w=2
> > 
> > might change this side-effect.
> > 
> 
> Has there been an update to this patch? The last I see, the conversation
> ended at the end of January, and there hasn't been a change in the
> mainline.

Peter Anvin was going to look at this with some of the Intel hardware
people to fully explore the ramifications of this change, we're waiting
on feedback from that.

> >> Your adjustments here make complete sense. Are you preparing it for
> submission in the near future?

I'll think about it, adding the extra barrier for decrement is of course
not really nice if not strictly required. And while it will not impact
x86 the weakly ordered archs will be affected.