linux-kernel - Re: [PATCH v3 02/10] zram: use zram->lock to protect zram_free

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Date:	Fri, 7 Jun 2013 17:05:34 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Jiang Liu <liuj97@...il.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Nitin Gupta <ngupta@...are.org>,
	Jerome Marchand <jmarchan@...hat.com>,
	Jiang Liu <jiang.liu@...wei.com>, devel@...verdev.osuosl.org,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v3 02/10] zram: use zram->lock to protect
 zram_free_page() in swap free notify path

On Fri, Jun 07, 2013 at 12:07:23AM +0800, Jiang Liu wrote:
> zram_slot_free_notify() is free-running without any protection from
> concurrent operations. So there are race conditions between
> zram_bvec_read()/zram_bvec_write() and zram_slot_free_notify(),
> and possible consequences include:
> 1) Trigger BUG_ON(!handle) on zram_bvec_write() side.
> 2) Access to freed pages on zram_bvec_read() side.
> 3) Break some fields (bad_compress, good_compress, pages_stored)
>    in zram->stats if the swap layer makes concurrently call to
>    zram_slot_free_notify().
> 
> So enhance zram_slot_free_notify() to acquire writer lock on zram->lock
> before calling zram_free_page().
> 

If someone try to read/write *active* swap device via opening
block device file(it's not sane but we couldn't prevent it),
the race between zram_slot_free_notify and zram_bvec_[read|write] can happen.
In such case, following problem for example can happen.

1. xxx
2. xxx
3. xxx

So this patch closes the race with zram->lock write-side lock.

> Signed-off-by: Jiang Liu <jiang.liu@...wei.com>
> Cc: stable@...r.kernel.org

Acked-by: Minchan Kim <minchan@...nel.org>

But please rewrite the description.

-- 
Kind regards,
Minchan Kim
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