Kernel 4.4.0 KVM guest on Btrfs locks up on snapshotting

[Roman Mamedov <rm@romanrm.net>]

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Hello,

I'm facing a strange issue:

Starting with the kernel 4.4.0, a KVM guest stored on a Btrfs filesystem,
if it's using the "virtio-scsi" disk backend, will hard lock-up instantly,
as soon as the Btrfs subvolume which contains its backing file is snapshotted.

There's nothing in dmesg neither on the guest, nor on the host;
the KVM process can be killed from the host just fine and then restarted,
so it doesn't seem to be a kernel-side deadlock of any sort.

KVM disk controller which exhibits the problem:

 -device virtio-scsi-pci,id=scsi -device scsi-hd,drive=hd \

The alternative which works fine:

 -device ide-hd,drive=hd,bus=ide.0 \

The disk device line is common to both cases:

 -drive if=none,id=hd,cache=writeback,aio=threads,format=raw,file=$NAME.img,discard=unmap,detect-zeroes=unmap \

Also tried aio=native with the problematic variant, no change.

Both variants work fine and unaffected by snapshotting on kernel 3.18.25.

Any ideas?

-- 
With respect,
Roman

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