linux-kernel - Oops in d_instantiate (fs/cache.c)

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Date:	Sat, 3 Dec 2011 13:19:22 +0100
From:	Stijn Devriendt <highguy@...il.com>
To:	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Oops in d_instantiate (fs/cache.c)

Hi all,

I've had 2 occasions where udev crashed during bootup.
The second time carried a kernel log where the following line
"BUG_ON(!list_empty(&entry->d_alias))"
in d_instantiate is triggered when udev is attempting to
create a symlink in /dev (which is tmpfs/shmem).

I've tried reproducing this by doing as udev does:
- create temporary symlink
- move temporary symlink into place
in a tight loop (multiple processes) while multiple
other processes were removing the symlink in a
tight loop.
A third script was flushing the dentry/inode cache
every so often using drop_caches.
All to no avail.

I've been digging around in the kernel sources,
but I'm not sure what the d_alias field means
and what the actual case is the BUG_ON is
meant to catch. I'd like to be able to find a way
to reproduce this, because so far it's happened only
twice in 2 weeks over multiple systems doing
many reboots in a testing setup.
Can someone explain this to me in short?

Thanks,
Stijn
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