linux-kernel - Re: mips octeon memory model questions

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Message-ID: <20140204190535.GC5002@laptop.programming.kicks-ass.net>
Date:	Tue, 4 Feb 2014 20:05:35 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	David Daney <ddaney@...iumnetworks.com>,
	Ralf Baechle <ralf@...ux-mips.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	linux-mips <linux-mips@...ux-mips.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Will Deacon <will.deacon@....com>
Subject: Re: mips octeon memory model questions

On Tue, Feb 04, 2014 at 10:58:40AM -0800, Linus Torvalds wrote:
> On Tue, Feb 4, 2014 at 10:41 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > Still doesn't make sense, because if we need the first sync to stop
> > writes from being re-ordered with the ll-sc, we also need the second
> > sync to avoid the same.
> 
> Presumably octeon doesn't do speculative writes, only *buffered* writes.

Speculative writes are bad.. :-)

> So writes move down, not up.

Right, but the ll-sc store might move down over a later store. Say
because the ll-sc needs to first get exclusive ownership of the
cacheline where the later store would be to an already owned line.


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