linux-kernel - Re: kdevtmpfs oops since yesterdays vfs merge

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Date:	Mon, 25 Jul 2011 03:44:44 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: kdevtmpfs oops since yesterdays vfs merge

On Sun, Jul 24, 2011 at 09:56:12PM -0400, Dave Jones wrote:

> [    7.760774] dracut: luksOpen /dev/sda2 luks-b5a1fb36-5672-4191-a260-e3f389eb0bb6
> [   14.787158] nodename: dm-0
> [   15.082391] nodename: dm-0
> 
> 
> when it triggers the bug_on(), it's that second nodename that is garbage.

Interesting...  The next experiment would be to stick BUG_ON(!req.dev)
into devtmpfs_create_node() right after the assigment to that field.

We couldn't be hit by the lack of barriers here, could we?  Store to
req.dev happens before spin_unlock(&req_lock), so by the time when
that request is seen by loop in devtmpfsd() and passed to handle() it
should be seen - we have grabbed req_lock, found a pointer to req, dropped
req_lock and called handle().  Should've been enough...

Might be interesting to print &req from devtmpfs_create_node(), both on
entry and on exit, and print req right before the call of handle()...

Incidentally, that disassembly shows one really ugly thing - offset of
->devt in struct device is 0x3c0.  IOW, each of those suckers eats a
kilobyte... ;-/
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