linux-kernel - Re: [PATCH: 2.6.20-rc4-mm1] JFS: Avoid deadlock introduced by explicit I/O plugging

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Date:	Thu, 18 Jan 2007 10:18:47 +1100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Dave Kleikamp <shaggy@...ux.vnet.ibm.com>
Cc:	JFS Discussion <jfs-discussion@...ts.sourceforge.net>,
	fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Nick Piggin <nickpiggin@...oo.com.au>
Subject: Re: [PATCH: 2.6.20-rc4-mm1] JFS: Avoid deadlock introduced by explicit  I/O plugging

On Wed, Jan 17 2007, Dave Kleikamp wrote:
> Jens,
> Can you please take a look at this patch, and if you think it's sane,
> add it to your explicit i/o plugging patchset?  Would it make sense in
> any of these paths to use io_schedule() instead of schedule()?

I'm glad you bring that up, actually. One of the "downsides" of the new
unplugging is that it really requires anyone waiting for IO in a path
like the file system or device driver to use io_schedule() instead of
schedule() to get the blk_replug_current_nested() done to avoid
deadlocks. While it is annoying that it could introduce some deadlocks
until we get things fixed it, I do consider it a correctness fix even in
the generic kernel, as you are really waiting for IO and as such should
use io_schedule() in the first place.

Perhaps I should add a WARN_ON() check for this to catch these bugs
upfront.

> I hadn't looked at your patchset until I discovered that jfs was easy to
> hang in the -mm kernel.  I think jfs may be able to add explicit
> plugging and unplugging in a couple of places, but I'd like to fix the
> hang right away and take my time with any later patches.

Can you try io_schedule() and verify that things just work?

-- 
Jens Axboe

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