linux-kernel - Re: [RFC][PATCH v2] slub: Keep page and object in sync in slab_alloc

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Message-ID: <0000013c4eec1cbd-fb392b9f-b39e-4bcf-a043-2fa76fb8d35a-000000@email.amazonses.com>
Date:	Fri, 18 Jan 2013 18:29:22 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	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 v2] slub: Keep page and object in sync in
 slab_alloc_node()

On Fri, 18 Jan 2013, Steven Rostedt wrote:

> On Fri, 2013-01-18 at 10:04 -0500, Steven Rostedt wrote:
>
> Just to be more complete:
>
> > 	CPU0			CPU1
> > 	----			----
> 			c = __this_cpu_ptr(s->cpu_slab);
> 			<migrates to CPU0>
>
> > <cpu fetches c->page>
> 			<another task>
>
> > 			updates c->tid

We can avoid the above scenario by doing a cpu local fetch.

i.e.
`	tid = this_cpu_read(s->cpu_slab->tid);


> > 			updates c->page
> > 			updates c->freelist
> > <cpu fetches c->tid>
> > <cpu fetches c->freelist>
> >
> >   node_match() succeeds even though
> >     current c->page wont
> >
>
>  <migrates back to CPU 1>
>
> >  this_cpu_cmpxchg_double() only tests
> >    the object (freelist) and tid, both which
> >    will match, but the page that was tested
> >    isn't the right one.
> >
>
> Yes, it's very unlikely, but we are in the business of dealing with the
> very unlikely. That's because in our business, the very unlikely is very
> likely. Damn, I need to buy a lotto ticket!

Well, the consequence would be that an object from another node than
desired will be allocated. Not that severe of an issue.
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