linux-kernel - Re: [PATCH 1/2] Add partition resize function to BLKPG ioctl

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Message-ID: <4F00D4D1.7020506@cfl.rr.com>
Date:	Sun, 01 Jan 2012 16:49:05 -0500
From:	Phillip Susi <psusi@....rr.com>
To:	Maxim Patlasov <maxim.patlasov@...il.com>
CC:	vgoyal@...hat.com, joe@...ches.com, kzak@...hat.com,
	linux-kernel@...r.kernel.org, jaxboe@...ionio.com
Subject: Re: [PATCH 1/2] Add partition resize function to BLKPG ioctl

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On 12/29/2011 07:09 PM, Phillip Susi wrote:
> Good points.  I also noticed that the read/write functions were only
> being called when not holding the mutex.  If anyone is touching
> nr_sects without the mutex, then everyone must use the read/write
> functions, whether they hold the mutex or not.  Otherwise, a mutex
> holder that touches it directly will race with a non mutex holder
> using the seqcounter.  Vivek, rather than fix the rest of the
> references to nr_sects to use the read/write functions, why not just
> fix the few sites that were accessing it without the mutex to take
> the mutex fist instead of using a seqcounter?

I've been wondering why device-mapper and loop have been able to change device size for years, yet didn't add a seqcounter to do this.  It looks to me like there are plenty of places ( like xen-blkback ) that either call get_capacity() or directly reference nr_sects without holding the bd_mutex.  Does this mean that device-mapper or loop changing capacity on i386 with large block support already has a race condition?
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