[libvirt] [PATCH] virNetDevVethCreate: Serialize callers

Eric Blake eblake at redhat.com
Tue Feb 25 23:40:12 UTC 2014


On 02/25/2014 09:07 AM, Michal Privoznik wrote:
> Consider dozen of LXC domains, each of them having this type of interface:
> 
>     <interface type='network'>
>       <mac address='52:54:00:a7:05:4b'/>
>       <source network='default'/>
>     </interface>
> 
> When starting these domain in parallel, all workers may meet in
> virNetDevVethCreate() where a race starts. Race over allocating veth
> pairs because allocation requires two steps:
> 
>   1) find first nonexistent '/sys/class/net/vnet%d/'
>   2) run 'ip link add ...' command
> 
> Now consider two threads. Both of them find N as the first unused veth
> index but only one of them succeeds allocating it. The other one fails.
> For such cases, we are running the allocation in a loop with 10 rounds.
> However this is very flaky synchronization. It should be rather used
> when libvirt is competing with other process than when libvirt threads
> fight each other. Therefore, internally we should use mutex to serialize
> callers, and do the allocation in loop (just in case we are competing
> with a different process). By the way we have something similar already
> since 1cf97c87.
> 
> Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
> ---
>  src/util/virnetdevveth.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 

ACK

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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