linux-kernel - Re: [PATCH v2] introduce atomic_pointer to fix a race condition in cancelable mcs spinlocks

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Message-ID: <CA+55aFzwrqkwD=G=w2m7TdXTpsMFjV7XUMXv5DMPGr=5x5YGCQ@mail.gmail.com>
Date:	Mon, 2 Jun 2014 10:09:35 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mikulas Patocka <mpatocka@...hat.com>,
	"James E.J. Bottomley" <jejb@...isc-linux.org>,
	Helge Deller <deller@....de>,
	John David Anglin <dave.anglin@...l.net>,
	Parisc List <linux-parisc@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	"Vinod, Chegu" <chegu_vinod@...com>,
	Waiman Long <Waiman.Long@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Davidlohr Bueso <davidlohr@...com>,
	Peter Anvin <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	Jason Low <jason.low2@...com>
Subject: Re: [PATCH v2] introduce atomic_pointer to fix a race condition in
 cancelable mcs spinlocks

On Mon, Jun 2, 2014 at 9:25 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>
> And I can't say I'm a particular fan of these ops either, as alternative
> I'm almost inclined to just exclude parisc from using opt spinning.

Please do.

There is no way in hell that we should introduce a magic new
atomic_pointer thing for parisc. And the idea somebody had to change
ACCESS_ONCE() for PA-RISC (I'm not going to go back to find who to
blame) is just horribly wrong too, since it's not even necessary for
any normal use: the special "load-and-store-zero" instruction isn't
actually used for "real" data, it's used only for the special
spinlocks afaik, so doing it for all ACCESS_ONCE() users would be
wrong even on PA-RISC. For any normal data, the usual "just load the
value, making sure the compiler doesn't reload it" is perfectly fine -
even on PA-RISC.

Now, if PA-RISC was a major architecture, we'd have to figure this
out. But as it is, PA-RISC is just about the shittiest RISC ever
invented (with original sparc being a strong contender), and let's
face it, nobody really uses it.  It's a "fun project", but it is not
something that we should use to mess up either ACCESS_ONCE() or the
MCS locks.

Just make PA-RISC use its own locks, not any of the new fancy ones.

              Linus
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