linux-ext4 - Re: [PATCH 2/4] ext3: Fix possible deadlock between ext3_truncate() and ext3_get

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Date:	Fri, 14 Aug 2009 16:41:05 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jan Kara <jack@...e.cz>
Cc:	linux-ext4@...r.kernel.org, tytso@....edu, jack@...e.cz
Subject: Re: [PATCH 2/4] ext3: Fix possible deadlock between ext3_truncate()
 and ext3_get_blocks()

On Fri, 14 Aug 2009 14:26:10 +0200
Jan Kara <jack@...e.cz> wrote:

> During truncate we are sometimes forced to start a new transaction as the
> amount of blocks to be journaled is both quite large and hard to predict. So
> far we restarted a transaction while holding truncate_mutex and that violates
> lock ordering because truncate_mutex ranks below transaction start (and it
> can lead to a real deadlock with ext3_get_blocks() allocating new blocks
> from ext3_writepage()).
> 
> Luckily, the problem is easy to fix: We just drop the truncate_mutex before
> restarting the transaction and acquire it afterwards. We are safe to do this as
> by the time ext3_truncate() is called, all the page cache for the truncated
> part of the file is dropped and so writepage() cannot come and allocate new
> blocks in the part of the file we are truncating. The rest of writers is
> stopped by us holding i_mutex.

For ext2 we have the comment:

	/*
	 * truncate_mutex is for serialising ext2_truncate() against
	 * ext2_getblock().  It also protects the internals of the inode's
	 * reservation data structures: ext2_reserve_window and
	 * ext2_reserve_window_node.
	 */

does truncate_mutex also protect ext3's reservation data?  If so, is
that impacted by this patch?

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