Re: [PATCH] Fix race condition reconnecting to vms & loading configs

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On Mon, Oct 28, 2013 at 11:52:50AM +0000, Daniel P. Berrange wrote:
> From: "Daniel P. Berrange" <berrange@xxxxxxxxxx>
> 
> The following sequence
> 
>  1. Define a persistent QMEU guest
>  2. Start the QEMU guest
>  3. Stop libvirtd
>  4. Kill the QEMU process
>  5. Start libvirtd
>  6. List persistent guets
> 
> At the last step, the previously running persistent guest
> will be missing. This is because of a race condition in the
> QEMU driver startup code. It does
> 
>  1. Load all VM state files
>  2. Spawn thread to reconnect to each VM
>  3. Load all VM config files
> 
> Only at the end of step 3, does the 'virDomainObjPtr' get
> marked as "persistent". There is therefore a window where
> the thread reconnecting to the VM will remove the persistent
> VM from the list.
> 
> The easy fix is to simply switch the order of steps 2 & 3.
> 
> Signed-off-by: Daniel P. Berrange <berrange@xxxxxxxxxx>
> ---
>  src/qemu/qemu_driver.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
> index c613967..9c3daad 100644
> --- a/src/qemu/qemu_driver.c
> +++ b/src/qemu/qemu_driver.c
> @@ -816,8 +816,6 @@ qemuStateInitialize(bool privileged,
>  
>      conn = virConnectOpen(cfg->uri);
>  
> -    qemuProcessReconnectAll(conn, qemu_driver);
> -
>      /* Then inactive persistent configs */
>      if (virDomainObjListLoadAllConfigs(qemu_driver->domains,
>                                         cfg->configDir,
> @@ -828,6 +826,7 @@ qemuStateInitialize(bool privileged,
>                                         NULL, NULL) < 0)
>          goto error;
>  
> +    qemuProcessReconnectAll(conn, qemu_driver);
>  
>      virDomainObjListForEach(qemu_driver->domains,
>                              qemuDomainSnapshotLoad,

Self-NACK.

The qemuProcessReconnectAll() method assumes that *only* live configs have
been loaded. Reordering this, makes it try to connect to all VMs, even
those which were never live.


Daniel
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