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Re: [libvirt] [PATCH] qemu_migrate: use nested job when adding NBD to cookie



On 08.04.2015 08:51, Michael Chapman wrote:
> 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>
> ---
>  src/qemu/qemu_migration.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 

ACKed and pushed.

Michal


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