linux-kernel - Re: [PATCH 4/4] Staging: zram: Replace mutex lock by a R/W semaphore

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Date:	Fri, 10 Jun 2011 09:46:46 -0700
From:	Nitin Gupta <ngupta@...are.org>
To:	Jerome Marchand <jmarchan@...hat.com>
CC:	Greg Kroah-Hartman <gregkh@...e.de>,
	Linux Kernel List <linux-kernel@...r.kernel.org>,
	Robert Jennings <rcj@...ux.vnet.ibm.com>,
	Jeff Moyer <jmoyer@...hat.com>
Subject: Re: [PATCH 4/4] Staging: zram: Replace mutex lock by a R/W semaphore

On 06/10/2011 06:28 AM, Jerome Marchand wrote:
> Currently, nothing protects zram table from concurrent access.
> For instance, ZRAM_UNCOMPRESSED bit can be cleared by zram_free_page()
> called from a concurrent write between the time ZRAM_UNCOMPRESSED has
> been set and the time it is tested to unmap KM_USER0 in
> zram_bvec_write(). This ultimately leads to kernel panic.
>
> Also, a read request can occurs when the page has been freed by a
> running write request and before it has been updated, leading to
> zero filled block being incorrectly read and "Read before write"
> error message.
>
> This patch replace the current mutex by a rw_semaphore. It extends
> the protection to zram table (currently, only compression buffers are
> protected) and read requests (currently, only write requests are
> protected).
>

These locking issues are probably remnants of earlier versions where
zram could be used only as a swap disks under which case it was not
possible for a read and write on the same sector (page) to happen
concurrently and thus there was no need to protect the table.


> Signed-off-by: Jerome Marchand<jmarchan@...hat.com>

Reviewed-by: Nitin Gupta <ngupta@...are.org>

Thanks,
Nitin
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