linux-ext4 - Re: [RFC] ext3 freeze feature

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Message-Id: <01c801c861ae$c2349990$41a8400a@bsd.tnes.nec.co.jp>
Date:	Mon, 28 Jan 2008 22:07:38 +0900
From:	"Takashi Sato" <t-sato@...jp.nec.com>
To:	"David Chinner" <dgc@....com>
Cc:	<linux-kernel@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
	<linux-ext4@...r.kernel.org>
Subject: Re: [RFC] ext3 freeze feature

Hi,

Thank you for your comments.

> That's inherently unsafe - you can have multiple unfreezes
> running in parallel which seriously screws with the bdev semaphore
> count that is used to lock the device due to doing multiple up()s
> for every down.
> 
> Your timeout thingy guarantee that at some point you will get
> multiple up()s occuring due to the timer firing racing with
> a thaw ioctl. 
> 
> If this interface is to be more widely exported, then it needs
> a complete revamp of the bdev is locked while it is frozen so
> that there is no chance of a double up() ever occuring on the
> bd_mount_sem due to racing thaws.....

My patch has the race condition as you said.
I will fix it.

Cheers, Takashi 

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