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qemuMigrationCookieAddNBD is usually called from within an async
MIGRATION_OUT or MIGRATION_IN job, so it needs to start a nested job.
(The one exception is during the Begin phase when change protection
isn't enabled, but qemuDomainObjEnterMonitorAsync will behave the same
as qemuDomainObjEnterMonitor in this case.)
This bug was encountered with a libvirt client that repeatedly queries
the disk mirroring block job info during a migration. If one of these
queries occurs just as the Perform migration cookie is baked, libvirt
crashes.
Relevant logs are as follows:
6701: warning : qemuDomainObjEnterMonitorInternal:1544 : This thread seems to be the async job owner; entering monitor without asking for a nested job is dangerous
[1] 6701: info : qemuMonitorSend:972 : QEMU_MONITOR_SEND_MSG: mon=0x7fefdc004700 msg={"execute":"query-block","id":"libvirt-629"}
[2] 6699: info : qemuMonitorIOWrite:503 : QEMU_MONITOR_IO_WRITE: mon=0x7fefdc004700 buf={"execute":"query-block","id":"libvirt-629"}
[3] 6704: info : qemuMonitorSend:972 : QEMU_MONITOR_SEND_MSG: mon=0x7fefdc004700 msg={"execute":"query-block-jobs","id":"libvirt-630"}
[4] 6699: info : qemuMonitorJSONIOProcessLine:203 : QEMU_MONITOR_RECV_REPLY: mon=0x7fefdc004700 reply={"return": [...], "id": "libvirt-629"}
6699: error : qemuMonitorJSONIOProcessLine:211 : internal error: Unexpected JSON reply '{"return": [...], "id": "libvirt-629"}'
At [1] qemuMonitorBlockStatsUpdateCapacity sends its request, then waits
on mon->notify. At [2] the request is written out to the monitor socket.
At [3] qemuMonitorBlockJobInfo sends its request, and also waits on
mon->notify. The reply from the first request is received at [4].
However, qemuMonitorJSONIOProcessLine is not expecting this reply since
the second request hadn't completed sending. The reply is dropped and an
error is returned.
qemuMonitorIO signals mon->notify twice during its error handling,
waking up both of the threads waiting on it. One of them clears mon->msg
as it exits qemuMonitorSend; the other crashes:
qemuMonitorSend (mon=0x7fefdc004700, msg=<value optimized out>) at qemu/qemu_monitor.c:975
975 while (!mon->msg->finished) {
(gdb) print mon->msg
$1 = (qemuMonitorMessagePtr) 0x0
Signed-off-by: Michael Chapman <mike@very.puzzling.org>
LibVirt : simple API for virtualization
Libvirt is a C toolkit to interact with the virtualization capabilities
of recent versions of Linux (and other OSes). It is free software
available under the GNU Lesser General Public License. Virtualization of
the Linux Operating System means the ability to run multiple instances of
Operating Systems concurrently on a single hardware system where the basic
resources are driven by a Linux instance. The library aim at providing
long term stable C API initially for the Xen paravirtualization but
should be able to integrate other virtualization mechanisms if needed.
Daniel Veillard <veillard@redhat.com>