linux-kernel - Re: [PATCH -tip 2/3] sched/wake

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Message-ID: <20150915141439.GE16853@twins.programming.kicks-ass.net>
Date:	Tue, 15 Sep 2015 16:14:39 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Davidlohr Bueso <dave@...olabs.net>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH -tip 2/3] sched/wake_q: Relax to acquire semantics

On Tue, Sep 15, 2015 at 07:09:22AM -0700, Paul E. McKenney wrote:
> On Tue, Sep 15, 2015 at 02:48:00PM +0200, Peter Zijlstra wrote:
> > On Tue, Sep 15, 2015 at 05:41:42AM -0700, Paul E. McKenney wrote:
> > > > Never mind, the PPC people will implement this with lwsync and that is
> > > > very much not transitive IIRC.
> > > 
> > > I am probably lost on context, but...
> > > 
> > > It turns out that lwsync is transitive in special cases.  One of them
> > > is a series of release-acquire pairs, which can extend indefinitely.
> > > 
> > > Does that help in this case?
> > 
> > Probably not, but good to know. I still don't think we want to rely on
> > ACQUIRE/RELEASE being transitive in general though.
> 
> OK, I will bite...  Why not?

It would mean us reviewing all archs (again) and documenting it I
suppose. Which is of course entirely possible.

That said, I don't think the case at hand requires it, so lets postpone
this for now ;-)
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