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

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Message-ID: <alpine.LRH.2.02.1406021622200.5874@file01.intranet.prod.int.rdu2.redhat.com>
Date:	Mon, 2 Jun 2014 16:46:32 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Peter Zijlstra <peterz@...radead.org>,
	"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, 2 Jun 2014, Linus Torvalds wrote:

> 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

And what else do you want to do?

Peter Zijlstra said "I've been using xchg() and cmpxchg() without such 
consideration for quite a while." - so it basically implies that the 
kernel is full of such races, mcs_spinlock is just the most visible one 
that crashes the kernel first.

It's not only parisc - tile32, arc, metag (maybe hexagon) are broken too, 
because they don't have cmpxchg in hardware.

We have atomic_t, atomic64_t, atomic_long_t that can be sanely used even 
on architectures without hardware cmpxchg - so I ask - why can't we have 
atomic_pointer_t with the same semantics? (pointer type conversion issues 
can be solved, as it is done in the PATCH v2)

Mikulas
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