Messages in this thread | | | Date | Mon, 11 Jun 2007 18:47:05 -0400 | From | Mark Lord <> | Subject | Re: ext3fs: umount+sync not enough to guarantee metadata-on-disk |
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Jan Kara wrote: >> Andrew Morton wrote: >>> On Thu, 07 Jun 2007 12:11:58 -0400 >>> Chuck Ebbert <cebbert@redhat.com> 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?
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