linux-ext4 - Possible race between direct IO and JBD?

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Date:	Fri, 25 Apr 2008 16:38:23 -0700
From:	Mingming Cao <cmm@...ibm.com>
To:	akpm@...ux-foundation.org
Cc:	pbadari@...ibm.com, Jan Kara <jack@...e.cz>,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Possible race between direct IO and JBD?

Hi,

While looking at a bug related to direct IO returns to EIO, after
looking at the code, I found there is a window that
try_to_free_buffers() from direct IO could race with JBD, which holds
the reference to the data buffers before journal_commit_transaction()
ensures the data buffers has reached to the disk.

A little more detail: to prepare for direct IO, generic_file_direct_IO()
calls invalidate_inode_pages2_range() to invalidate the pages in the
cache before performaning direct IO.  invalidate_inode_pages2_range()
tries to free the buffers via try_to free_buffers(), but sometimes it
can't, due to the buffers is possible still on some transaction's
t_sync_datalist or t_locked_list waiting for
journal_commit_transaction() to process it. 

Currently Direct IO simply returns EIO if try_to_free_buffers() finds
the buffer is busy, as it has no clue that JBD is referencing it.

Is this a known issue and expected behavior? Any thoughts?

Mingming

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