linux-ext4 - Re: [PATCH 1/5] ext4: Avoid races caused by on-line resizing and SMP memory reordering

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Message-ID: <20090428171445.GD24043@mit.edu>
Date:	Tue, 28 Apr 2009 13:14:45 -0400
From:	Theodore Tso <tytso@....edu>
To:	Jan Kara <jack@...e.cz>
Cc:	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	Linux Kernel Developers List <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@...radead.org>,
	Al Viro <viro@...IV.linux.org.uk>
Subject: Re: [PATCH 1/5] ext4: Avoid races caused by on-line resizing and
	SMP memory reordering

On Tue, Apr 28, 2009 at 06:23:59PM +0200, Jan Kara wrote:
>   Ouch... Hmm, smp_rmb() isn't completely free and mainly it's a bit
> ugly and prone to errors (I'm afraid next time someone changes the
> allocation code, we miss some barriers again)... so.. Maybe a stupid
> idea but wouldn't it be easier to solve the online resize like: freeze
> the filesystem, do all the changes required for extend, unfreeze the
> filesystem?

Eric suggested a helper function when reading from s_groups_count.
That would take care of the code maintenance headache.  One problem
with using freeze/thaw is it won't work without a journal, and we do
support filesystems without journals in ext4.  (Probably the best
solution for netbooks with crapola SSD's.)

As far as smb_rmb() not being free, it's essentially free for
x86/x86_64 platforms.  Is it really that costly on other
architectures?

						- Ted
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