linux-kernel - Re: [PATCH] llist: Fix missing lockless

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Message-ID: <20150208011204.GB10472@omega>
Date:	Sun, 8 Feb 2015 14:12:04 +1300
From:	Michael Cree <mcree@...on.net.nz>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	linux-alpha@...r.kernel.org, Richard Henderson <rth@...ddle.net>,
	Ivan Kokshaysky <ink@...assic.park.msu.ru>,
	Matt Turner <mattst88@...il.com>,
	Huang Ying <ying.huang@...el.com>,
	linux-kernel@...r.kernel.org,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	David Howells <dhowells@...hat.com>,
	Pranith Kumar <bobby.prani@...il.com>, stable@...r.kernel.org
Subject: Re: [PATCH] llist: Fix missing lockless_dereference()

On Sun, Feb 08, 2015 at 08:59:41AM +0800, Greg KH wrote:
> On Sun, Feb 08, 2015 at 01:47:29PM +1300, Michael Cree wrote:
> > On Sat, Feb 07, 2015 at 10:30:44PM +0000, Mathieu Desnoyers wrote:
> > > > On Fri, Feb 06, 2015 at 09:08:21PM -0500, Mathieu Desnoyers wrote:
> > > > > A lockless_dereference() appears to be missing in llist_del_first().
> > > > > It should only matter for Alpha in practice.
> > 
> > What could one anticipate to be the symptoms of such a missing
> > lockless_dereference()?
> > 
> > The Alpha kernel is behaving pretty well provided one builds a machine
> > specific kernel and UP.  When running an SMP kernel some packages
> > (most notably the java runtime, but there are a few others) occasionally
> > lock up in a pthread call --- could be a problem in libc rather then the
> > kernel.
> 
> Hm, if only UP alpha needs to be supported, odds are we could rip a lot
> of odd stuff out of the kernel that deals with memory barriers and other
> nasty locking things that the Alpha requires.
> 
> Would that be ok?  Or is someone somewhere going to want to be running a
> SMP kernel on Alpha in the future?

I am running an SMP kernel on a 3-cpu Alpha system; it mostly works
just fine.

I was just noting that there is something up with java---it locks up
occassionally in a pthread call, and there are a few other packages
that occasionally fail in test suites when being built under an SMP
kernel but always pass when built under an UP kernel which suggests
there is a little buglet somewhere in the SMP code, either in the
kernel or in libc.

Running an SMP system for the Debian Alpha build daemon at debian-ports
is really useful for keeping up with the other architectures.

Cheers
Michael.
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