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

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Message-ID: <20140602200525.GD13930@laptop.programming.kicks-ass.net>
Date:	Mon, 2 Jun 2014 22:05:25 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@...com>
Cc:	Mikulas Patocka <mpatocka@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	jejb@...isc-linux.org, deller@....de,
	John David Anglin <dave.anglin@...l.net>,
	linux-parisc@...r.kernel.org, linux-kernel@...r.kernel.org,
	paulmck@...ux.vnet.ibm.com, chegu_vinod@...com, tglx@...utronix.de,
	riel@...hat.com, akpm@...ux-foundation.org, davidlohr@...com,
	hpa@...or.com, andi@...stfloor.org, aswin@...com,
	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 02, 2014 at 01:33:34PM -0400, Waiman Long wrote:
> On 06/02/2014 12:30 PM, Peter Zijlstra wrote:
> >On Mon, Jun 02, 2014 at 06:25:25PM +0200, Peter Zijlstra wrote:
> >>I'm almost inclined to just exclude parisc from using opt spinning.
> >>
> >>That said, this patch still doesn't address the far more interesting
> >>problem of actually finding these issues for these few weird archs.
> >So why do these archs provide xchg() and cmpxchg() at all? Wouldn't it
> >be much simpler if archs that cannot sanely do this, not provide these
> >primitives at all?
> 
> I believe xchg() and cmpxchg() are used in quite a number of places within
> the generic kernel code. So kernel compilation will fail if those APIs
> aren't provided by an architecture.

Yep.. so this is going to be painful for a while. But given their
(parisc, sparc32, metag-lock1) constraints, who knows how many of those
uses are actually broken.

So the question is, do you prefer subtly broken code or hard compile
fails? Me, I go for the compile fail.

In any case, this all goes towards what hpa said, what are the minimal
requirements we have for running Linux.
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