linux-ext4 - Re: [PATCH] ext4: Prevent race while waling extent tree

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Message-ID: <20121108215233.GB1282@lenny.home.zabbo.net>
Date:	Thu, 8 Nov 2012 13:52:33 -0800
From:	Zach Brown <zab@...hat.com>
To:	Lukas Czerner <lczerner@...hat.com>
Cc:	linux-ext4@...r.kernel.org, tytso@....edu
Subject: Re: [PATCH] ext4: Prevent race while waling extent tree

On Thu, Nov 08, 2012 at 12:08:49PM +0100, Lukas Czerner wrote:
> +		down_read(&EXT4_I(inode)->i_data_sem);
>  		error = ext4_ext_walk_space(inode, start_blk, len_blks,
>  					  ext4_ext_fiemap_cb, fieinfo);
> +		up_read(&EXT4_I(inode)->i_data_sem);

Can this deadlock?  ext4_ext_fiemap_cb() seems to be doing all kinds of
exciting things that might also try and acquire the i_data_sem, like
GFP_KERNEL allocs (reclaim -> writepage) and copying to userspace (mmap
fault -> readpage -> get blocks).

It seems like the safer fix is to broaden the sampling lock coverage to
include referencing all the extent data but to release it around the
callback.

No?

- z
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