linux-kernel - Re: [PATCH 0/2] revert changes to zcache_do

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Message-ID: <5037EAC8.6080403@linux.vnet.ibm.com>
Date:	Fri, 24 Aug 2012 15:57:44 -0500
From:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
To:	Minchan Kim <minchan@...nel.org>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nitin Gupta <ngupta@...are.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	devel@...verdev.osuosl.org, xiaoguangrong@...ux.vnet.ibm.com
Subject: Re: [PATCH 0/2] revert changes to zcache_do_preload()

On 08/23/2012 06:28 PM, Minchan Kim wrote:
> Okay, then, why do you think the patchsets are culprit?
> I didn't look the cleanup patch series of Xiao at that time
> so I can be wrong but as I just look through patch of
> "zcache: optimize zcache_do_preload", I can't find any fault
> because zcache_put_page checks irq_disable so we don't need
> to disable preemption so it seems that patch is correct to me.
> If the race happens by preemption, BUG_ON in zcache_put_page
> should catch it.
> 
> What do you mean? Do you have any clue in your mind?
> 
>         The commits undermine an assumption made by tmem_put() in
>         the cleancache path that preemption is disabled.

I do not have an explanation right now for why these commits
expose this issue.  The patch looks like it should be fine
to me, hence my Ack at the time.

I understand and agree with you that the zcache shim
functions zcache_put_page(), zcache_get_page(),
zcache_flush_page(), and zcache_flush_object() all disable
interrupts (or make sure that interrupts are already
disabled) which implicitly disables preemption.

I'm still trying to find root cause here.

Seth

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