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

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Message-ID: <4F21CA16.3020002@ubuntu.com>
Date:	Thu, 26 Jan 2012 16:48:06 -0500
From:	Phillip Susi <psusi@...ntu.com>
To:	Vivek Goyal <vgoyal@...hat.com>
CC:	Maxim Patlasov <maxim.patlasov@...il.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 1/26/2012 4:04 PM, Vivek Goyal wrote:
>> Another idea that I had but have not yet checked to see if it is
>>  actually feasible is to copy the struct genhd, change the size
>> of the copy, and replace the existing one since updating the
>> pointer will be atomic.
> 
> You will run into issues if somebody has a pointer stored to 
> genhd.

They are already kept in an RCU list which has the same problem.
Doesn't that deal with it by using reference counters, so the reader
can keep and use the pointer to the old structure just fine, and it
will be cleaned up when they release the reference.

> I think simpler thing would be to stick with sequence counter 
> approach which keeps read side lockless. We can fix other writers 
> of nr_sects over a period of time. If nobody has complained so
> far, that means we don't run into issues frequently and it is not a
> huge concern.

So you think the patch is fine the way it is?

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