linux-kernel - Re: [PATCH] percpu-refcount: fix usage of this_cpu

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Date:	Wed, 4 Jun 2014 18:56:14 +0200 (CEST)
From:	Sebastian Ott <sebott@...ux.vnet.ibm.com>
To:	Tejun Heo <tj@...nel.org>
cc:	Anatol Pomozov <anatol.pomozov@...il.com>,
	Benjamin LaHaise <bcrl@...ck.org>, linux-aio@...ck.org,
	Kent Overstreet <kmo@...erainc.com>,
	Gu Zheng <guz.fnst@...fujitsu.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] percpu-refcount: fix usage of this_cpu_ops (was Re:
 hanging aio process)

Hi,

On Wed, 4 Jun 2014, Tejun Heo wrote:
> On Wed, Jun 04, 2014 at 03:58:24PM +0200, Sebastian Ott wrote:
> > From 82295633cad58c7d6b9af4e470e3168ed43a6779 Mon Sep 17 00:00:00 2001
> > From: Heiko Carstens <heiko.carstens@...ibm.com>
> > Date: Wed, 4 Jun 2014 12:53:19 +0200
> > Subject: [PATCH] percpu-refcount: fix usage of this_cpu_ops
> > 
> > The percpu-refcount infrastructure uses the underscore variants of
> > this_cpu_ops in order to modify percpu reference counters.
> > (e.g. __this_cpu_inc()).
> > 
> > However the underscore variants do not atomically update the percpu
> > variable, instead they may be implemented using read-modify-write
> > semantics (more than one instruction).  Therefore it is only safe to
> > use the underscore variant if the context is always the same (process,
> > softirq, or hardirq). Otherwise it is possible to lose updates.
> > 
> > This problem is something that Sebastian has seen within the aio
> > subsystem which uses percpu refcounters both in process and softirq
> > context leading to reference counts that never dropped to zeroes; even
> > though the number of "get" and "put" calls matched.
> > 
> > Fix this by using the non-underscore this_cpu_ops variant which
> > provides correct per cpu atomic semantics and fixes the corrupted
> > reference counts.
> > 
> > Cc: Kent Overstreet <kmo@...erainc.com>
> > Cc: Tejun Heo <tj@...nel.org>
> > Cc: <stable@...r.kernel.org> # v3.11+
> > Reported-by: Sebastian Ott <sebott@...ux.vnet.ibm.com>
> > Signed-off-by: Heiko Carstens <heiko.carstens@...ibm.com>
> 
> Bah... should have caught it way back.  Sorry about that.  Appying to
> percpu/for-3.15-fixes with stable cc'd.
> 

In your git tree you've used me as the patch author but that should be
Heiko. Sorry if I didn't annotate this correctly - I just inlined the
git format-patch output..

Thanks,
Sebastian

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