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

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Message-ID: <CA+55aFwCy4-x02U6A-KbkPEpe_4pagsZ__ipBhe5HZOQt2vNmw@mail.gmail.com>
Date:	Mon, 2 Jun 2014 13:53:48 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Mikulas Patocka <mpatocka@...hat.com>
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, Jun 2, 2014 at 1:46 PM, Mikulas Patocka <mpatocka@...hat.com> wrote:
>
> 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.

.. so your whole argument is bogus, because it doesn't actually fix
anything else.

Now, something that *would* fix something else is (for example) to
just make "ACCESS_ONCE()" a rvalue so that you cannot use it for
assignments, and then trying to sort out what happens then. It's
possible that the "atomic_pointer_t" would be a part of the solution
to that "what happens then", but THERE IS NO WAY IN HELL we're adding
it for just one architecture and one use that doesn't warrant even
_existing_ on that architecture.

See what I'm saying?

You're not fixing the problem, you're fixing one unimportant detail
that isn't worth fixing that way.

               Linus
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