linux-ext4 - Re: [PATCH] ext4: fix potential deadlock in ext4_nonda

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Message-ID: <20120927180949.GA6862@thunk.org>
Date:	Thu, 27 Sep 2012 14:09:49 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Dmitry Monakhov <dmonakhov@...nvz.org>
Cc:	Eric Sandeen <sandeen@...hat.com>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	stable@...r.kernel.org
Subject: Re: [PATCH] ext4: fix potential deadlock in ext4_nonda_switch()

On Thu, Sep 27, 2012 at 09:18:30PM +0400, Dmitry Monakhov wrote:
> Hmm... even this '!' patch is still not good. I've got that complain
> WARNING: at fs/fs-writeback.c:1316 writeback_inodes_sb_nr+0x1a3/0x200()

This WARN_ON checking to make sure the s_umount mutex is grabbed.

What kernel are you applying this patch against?  Commit 14da92001:
"fs: Protect write paths by sb_start_write - sb_end_write" adds a call
to sb_start_write() in generic_file_aio_write() which means by the
time we call ext4_nonda_switch(), s_umount is grabbed, which probably
explains why I'm not seeing this --- 14da92001 was added to mainline
as of 3.6-rc1, and I'm guessing you're using an older kernel?

This is something we need to consider, though, since "ext4: fix
potential deadlock in ext4_nonda_switch()" is currently marked for
stable, but commit 14da92001 is not marked for stable.  So if this
commit gets backported w/o 14da92001, stable kernel users will see
this warning.

						- Ted
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