From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [PATCH v2] introduce atomic_pointer to fix a race condition in cancelable mcs spinlocks Date: Mon, 2 Jun 2014 22:05:25 +0200 Message-ID: <20140602200525.GD13930@laptop.programming.kicks-ass.net> References: <20140602162525.GH16155@laptop.programming.kicks-ass.net> <20140602163032.GI16155@laptop.programming.kicks-ass.net> <538CB56E.5010709@hp.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Mikulas Patocka , Linus Torvalds , jejb@parisc-linux.org, deller@gmx.de, John David Anglin , linux-parisc@vger.kernel.org, linux-kernel@vger.kernel.org, paulmck@linux.vnet.ibm.com, chegu_vinod@hp.com, tglx@linutronix.de, riel@redhat.com, akpm@linux-foundation.org, davidlohr@hp.com, hpa@zytor.com, andi@firstfloor.org, aswin@hp.com, scott.norton@hp.com, Jason Low To: Waiman Long Return-path: In-Reply-To: <538CB56E.5010709@hp.com> List-ID: List-Id: linux-parisc.vger.kernel.org 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.