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

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Date:	Fri, 9 Nov 2012 10:19:52 +0100 (CET)
From:	Lukáš Czerner <lczerner@...hat.com>
To:	Zach Brown <zab@...hat.com>
cc:	Lukas Czerner <lczerner@...hat.com>, linux-ext4@...r.kernel.org,
	tytso@....edu
Subject: Re: [PATCH] ext4: Prevent race while waling extent tree

On Thu, 8 Nov 2012, Zach Brown wrote:

> 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

Yeah, you're right. Having the lock around the whole
ext4_ext_walk_space() might deadlock. I'll fix this.

Thanks for noticing this!
-Lukas
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