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From: Larkin Lowrey <llowrey@nuclearwinter.com>
To: Btrfs BTRFS <linux-btrfs@vger.kernel.org>
Subject: Possible deadlock when writing
Date: Mon, 26 Nov 2018 13:18:06 -0500	[thread overview]
Message-ID: <61991cd9-13ba-1834-3112-0f10e925a48c@nuclearwinter.com> (raw)

I started having a host freeze randomly when running a 4.18 kernel. The 
host was stable when running 4.17.12.

At first, it appeared that it was only IO that was frozen since I could 
run common commands that were likely cached in RAM and that did not 
touch storage. Anything that did touch storage would freeze and I would 
not be able to ctrl-c it.

I noticed today, when it happened with kernel 4.19.2, that backups were 
still running and that the backup app could still read from the backup 
snapshot subvol. It's possible that the backups are still able to 
proceed because the accesses are all read-only and the snapshot was 
mounted with noatime so the backup process never triggers a write.

There never are any errors output to the console when this happens and 
nothing is logged. When I first encountered this back in Sept. I managed 
to record a few sysrq dumps and attached them to a redhat ticket. See 
links below.

https://bugzilla.redhat.com/show_bug.cgi?id=1627288
https://bugzilla.redhat.com/attachment.cgi?id=1482177

I do have several VMs running that have their image files nocow'd. 
Interestingly, all the VMs, except 1, seem to be able to write just 
fine. The one that can't has frozen completely and is the one that 
regularly generates the most IO.

Any ideas on how to debug this further?

--Larkin

             reply	other threads:[~2018-11-26 19:13 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-26 18:18 Larkin Lowrey [this message]
2018-12-01 15:15 ` Possible deadlock when writing Janos Toth F.
2018-12-01 23:21 Martin Bakiev
2018-12-03  9:36 ` Tony Lambiris

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