Re: [libvirt] [PATCH] nwfilter: fix lock order deadlock

Below is backtraces of two deadlocked threads: thread #1: virDomainConfVMNWFilterTeardown virNWFilterTeardownFilter lock updateMutex <------------ _virNWFilterTeardownFilter try to lock interface <---------- thread #2: learnIPAddressThread lock interface <------- virNWFilterInstantiateFilterLate try to lock updateMutex <---------- The problem is fixed by unlocking interface before calling virNWFilterInstantiateFilterLate to avoid updateMutex and interface ordering deadlocks. Otherwise we are going to instantiate the filter while holding interface lock, which will try to lock updateMutex, and if some other thread instantiating a filter in parallel is holding updateMutex and is trying to lock interface, both will deadlock. Also it is safe to unlock interface before virNWFilterInstantiateFilterLate because learnIPAddressThread stopped capturing packets and applied necessary rules on the interface, while instantiating a new filter doesn't require a locked interface. Signed-off-by: Maxim Nestratov <mnestratov(a)virtuozzo.com&gt; --- src/nwfilter/nwfilter_learnipaddr.c | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-) diff --git a/src/nwfilter/nwfilter_learnipaddr.c b/src/nwfilter/nwfilter_learnipaddr.c index 1adbadb..cfd92d9 100644 --- a/src/nwfilter/nwfilter_learnipaddr.c +++ b/src/nwfilter/nwfilter_learnipaddr.c @@ -611,6 +611,16 @@ learnIPAddressThread(void *arg) sa.data.inet4.sin_addr.s_addr = vmaddr; char *inetaddr; + /* It is necessary to unlock interface here to avoid updateMutex and + * interface ordering deadlocks. Otherwise we are going to + * instantiate the filter, which will try to lock updateMutex, and + * some other thread instantiating a filter in parallel is holding + * updateMutex and is trying to lock interface, both will deadlock. + * Also it is safe to unlock interface here because we stopped + * capturing and applied necessary rules on the interface, while + * instantiating a new filter doesn't require a locked interface.*/ + virNWFilterUnlockIface(req->ifname); + if ((inetaddr = virSocketAddrFormat(&sa)) != NULL) { if (virNWFilterIPAddrMapAddIPAddr(req->ifname, inetaddr) < 0) { VIR_ERROR(_("Failed to add IP address %s to IP address " @@ -636,11 +646,11 @@ learnIPAddressThread(void *arg) req->ifname, req->ifindex); techdriver->applyDropAllRules(req->ifname); + virNWFilterUnlockIface(req->ifname); } VIR_DEBUG("pcap thread terminating for interface %s", req->ifname); - virNWFilterUnlockIface(req->ifname); err_no_lock: virNWFilterDeregisterLearnReq(req->ifindex);