linux-kernel - Re: [BUG REPORT] ZSWAP: theoretical race condition issues

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Date:	Wed, 25 Sep 2013 16:31:08 +0800
From:	Bob Liu <lliubbo@...il.com>
To:	Weijie Yang <weijie.yang.kh@...il.com>
Cc:	Minchan Kim <minchan@...nel.org>, Bob Liu <bob.liu@...cle.com>,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Linux-MM <linux-mm@...ck.org>,
	Linux-Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [BUG REPORT] ZSWAP: theoretical race condition issues

On Wed, Sep 25, 2013 at 4:09 PM, Weijie Yang <weijie.yang.kh@...il.com> wrote:
> I think I find a new issue, for integrity of this mail thread, I reply
> to this mail.
>
> It is a concurrence issue either, when duplicate store and reclaim
> concurrentlly.
>
> zswap entry x with offset A is already stored in zswap backend.
> Consider the following scenario:
>
> thread 0: reclaim entry x (get refcount, but not call zswap_get_swap_cache_page)
>
> thread 1: store new page with the same offset A, alloc a new zswap entry y.
>   store finished. shrink_page_list() call __remove_mapping(), and now
> it is not in swap_cache
>

But I don't think swap layer will call zswap with the same offset A.

> thread 0: zswap_get_swap_cache_page called. old page data is added to swap_cache
>
> Now, swap cache has old data rather than new data for offset A.
> error will happen If do_swap_page() get page from swap_cache.
>

-- 
Regards,
--Bob
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