Re: [PATCH v3 02/10] zram: use zram->lock to protectzram_free_page() in swap free notify path

From: Minchan Kim
Date: Fri Jun 07 2013 - 04:05:46 EST

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@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx

Acked-by: Minchan Kim <minchan@xxxxxxxxxx>

But please rewrite the description.

Kind regards,
Minchan Kim
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