linux-kernel - Re: [RFC][PATCH v3] slub: Keep page and object in sync in slab_alloc

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Date:	Fri, 18 Jan 2013 14:20:35 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Christoph Lameter <cl@...ux.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>,
	Matt Mackall <mpm@...enic.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	RT <linux-rt-users@...r.kernel.org>,
	Clark Williams <clark@...hat.com>,
	John Kacur <jkacur@...il.com>,
	"Luis Claudio R. Goncalves" <lgoncalv@...hat.com>
Subject: Re: [RFC][PATCH v3] slub: Keep page and object in sync in
 slab_alloc_node()

On Fri, 2013-01-18 at 11:09 -0800, Eric Dumazet wrote:
> On Fri, 2013-01-18 at 18:40 +0000, Christoph Lameter wrote:
> 
> > The fetching of the tid is the only critical thing here. If the tid is
> > retrieved from the right cpu then the cmpxchg will fail if any changes
> > occured to freelist or the page variable.
> > 
> > The tid can be retrieved without disabling preemption through
> > this_cpu_read().
> 
> Strictly speaking, this_cpu_read() _does_ disable preemption.

I was thinking the same thing.

> 
> Of course, on x86, this_cpu_read() uses __this_cpu_read()


Looking at using this_cpu_read() on tid, I can't see what would break.
We still need the check for page being NULL in node_match(). I'm still
uneasy about it. Just because we can't see how it might break doesn't
mean that it wont. As we have already found a bit of bugs in the current
code. I'd feel more comfortable with the explicit preempt_disable(). And
this is coming from an -rt guy that tries to avoid preempt_disable().
But you're (Christoph) the maintainer.

I guess if you use this_cpu_read() you don't need that barrier anymore.

-- Steve


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