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



24.05.2016 13:56, Stefan Berger пишет:

On 05/24/2016 03:22 AM, Maxim Nestratov wrote:
20.04.2016 22:04, Stefan Berger пишет:

On 04/20/2016 12:06 PM, Laine Stump wrote:
On 04/19/2016 05:45 PM, Cole Robinson wrote:
On 04/09/2016 12:14 PM, Maxim Nestratov wrote:
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 virtuozzo com>
---
  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);

This looks sensible to me... the virNWFilterInstantiateFilterLate call tree certainly expects the iface to be unlocked at a certain point. This patch just
moves the unlock a bit earlier.

ACK, but maybe wait another day to see if anyone else wants to jump in, I'm not really familiar with this code (then again I doubt anyone watching the
list is deeply familiar with it either)

Stefan Berger is, but I don't think he watches the list closely these days. I just found the original message (somehow it had been tossed into my spam folder) and Replied-All to it with Stefan Cc'ed.

ACK. I agree to the change. It seems to be possible to release the lock earlier.

   Stefan



Shall I push it then?

Yes.

   Stefan


Pushed now. Thanks.

Maxim


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