Re: [libvirt] libvirtd live-locking on CTX_LOCK when doing 'virsh <domid> save /tmp/blah' with guest corrupting memory (on purpose).

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 08/04/15 15:47, Konrad Rzeszutek Wilk wrote:
> Hey Jim, Andrew, and Ian,
>
> This is libvirt v1.2.14 + three patches:
> c82a59b libxl: drop virDomainObj lock when destroying a domain
> a1c9d30 libxl: acquire a job when destroying a domain
> 5bd5406 libxl: Move job acquisition in libxlDomainStart to callers
>
> For fun I've set up an guest with PCI passthrough and tried to save it
> (HAHAH) with an disastrous result (xc_save_helper was stuck). Probably
> due to outstanding DMA operations wreaking havoc.

Outstanding DMA wont make any difference.  It isn't (and can't) be
reflected in the logdirty bitmap, so libxc simply wont know about it.

xc_save_helper is blocked because it has called back into the libxl with
the suspend_and_state() callback.

i.e. libxc has requested that libxl pause the domain, and that request
is still outstanding.


The vcpu trace from the very bottom shows that the guest has not yet
paused itself.  1 vcpu is blocked in the hypervisor while the other look
to be in some spinlock code.

~Andrew