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(a)redhat.com&gt; --- src/util/virnetdevveth.c | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff --git a/src/util/virnetdevveth.c b/src/util/virnetdevveth.c index 25eb282..e698ce2 100644 --- a/src/util/virnetdevveth.c +++ b/src/util/virnetdevveth.c @@ -39,6 +39,19 @@ /* Functions */ +virMutex virNetDevVethCreateMutex; + +static int virNetDevVethCreateMutexOnceInit(void) +{ + if (virMutexInit(&virNetDevVethCreateMutex) < 0) { + virReportSystemError(errno, "%s", _("unable to init mutex")); + return -1; + } + return 0; +} + +VIR_ONCE_GLOBAL_INIT(virNetDevVethCreateMutex); + static int virNetDevVethExists(int devNum) { int ret; @@ -117,6 +130,10 @@ int virNetDevVethCreate(char** veth1, char** veth2) * We might race with other containers, but this is reasonably * unlikely, so don't do too many retries for device creation */ + if (virNetDevVethCreateMutexInitialize() < 0) + return -1; + + virMutexLock(&virNetDevVethCreateMutex); #define MAX_VETH_RETRIES 10 for (i = 0; i < MAX_VETH_RETRIES; i++) { @@ -179,6 +196,7 @@ int virNetDevVethCreate(char** veth1, char** veth2) MAX_VETH_RETRIES); cleanup: + virMutexUnlock(&virNetDevVethCreateMutex); virCommandFree(cmd); VIR_FREE(veth1auto); VIR_FREE(veth2auto); -- 1.9.0