Re: ext3fs: umount+sync not enough to guarantee metadata-on-disk

From: Mark Lord
Date: Mon Jun 11 2007 - 18:47:20 EST


Jan Kara wrote:
Andrew Morton wrote:
On Thu, 07 Jun 2007 12:11:58 -0400
Chuck Ebbert <cebbert@xxxxxxxxxx> wrote:

On 06/07/2007 11:41 AM, Andrew Morton wrote:
mount /var/lib/mythtv -oremount,ro
sync
umount /var/lib/mythtv
Did this succeed? If the application is still truncating that file, the
umount should have failed.
Shouldn't sync should wait for truncate to finish?
I can't think of anything in there at present which would cause that to
happen, and it's not immediately obvious how we _could_ make it happen - we
have an inode which potentially has no dirty pages and which is itself
clean. The truncate can span multiple journal commits, so forcing a
journal commit in sync() won't necessarily block behind the truncate.

I guess we could ask sync to speculatively take and release every inode's
i_mutex or something. But even that would involve quite some hoop-jumping
due to those infuriating spinlock-protected list_heads on the superblock.

hmm.
Okay, I added more instrumentation and retested today.

Good and Bad.
The umount does indeed fail while the massive unlink is happening,
so I can just loop on that a few times before giving up.

But.. the earlier "remount,ro".. well.. I don't know what it does.
I did get it to lock up solid, though.. hung on the "remount,ro"
when issued during an unlink of a 15GB file. The disk I/O eventually
completes, and drives go idle, but the system remains hung inside
the remount,ro call.

Alt-sysrq-T was functioning, so I have some screen shots (.jpg) here:

http://rtr.ca/remount_ro/
Thanks for the traces.

That's definitely a bug.
Yes. We have a nice lock inversion there. ext3_remount() is called
with sb->s_lock held and waits for transaction to finish in
journal_lock_updates(). On the other hand ext3_orphan_del() is called
inside a transaction and tries to do lock_super()... Bad luck.


Peachy. Do you have enough knowledge here to generate a fix for this?
Maybe just have the remount break out, releasing all locks, and then loop and retry (or return -EBUSY?) when this happens?

Cheers
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