linux-kernel - Re: [PATCH] Introduce freeze_super and thaw

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Date:	Wed, 24 Mar 2010 10:52:52 +1100
From:	Nigel Cunningham <ncunningham@...a.org.au>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	Josef Bacik <josef@...hat.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, chris.mason@...cle.com, hch@....de
Subject: Re: [PATCH] Introduce freeze_super and thaw_super for the fsfreeze
 ioctl

Hi.

On 24/03/10 10:18, Al Viro wrote:
> On Wed, Mar 24, 2010 at 09:31:51AM +1100, Nigel Cunningham wrote:
>> /**
>>   * freeze_filesystems - lock all filesystems and force them into a
>> consistent
>>   * state
>>   * @which:      What combination of fuse&  non-fuse to freeze.
>>   */
>> void freeze_filesystems(int which)
>> {
>>          struct super_block *sb;
>>
>>          lockdep_off();
>>
>>          /*
>>           * Freeze in reverse order so filesystems dependant upon others are
>>           * frozen in the right order (eg. loopback on ext3).
>>           */
>
> [snip the horror]
>
> 	a) traversing superblock list without any locking whatsoever
> 	b) accessing superblock fields<....>
> 	c)<.........................>  without making sure that it's not
> going to disappear
> 	d) calling freeze_bdev() without any warranties that its argument
> is not going to be freed under you
> 	e) layering violations all over the place
> etc.
>
> I've stayed away from TuxOnIce flamefests and I've no idea how representative
> that snippet is, but if it *does* match the general code quality in there...
> Ouch.

Locking isn't necessary because of the freezing.

Regards,

Nigel
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