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Re: [libvirt] [PATCH v2 3/4] Add a mutex to serialize updates to firewall



On 01/27/2014 12:18 PM, Daniel P. Berrange wrote:
The nwfilter conf update mutex previously serialized
updates to the internal data structures for firewall
rules, and updates to the firewall itself. Since the

Hm, wasn't aware (anymore) of this double-purpose.

I also hadn't looked at this patch in the first round...

former is going to be turned into a read/write lock
instead of a mutex, a new lock is required to serialize
access to the firewall itself.

With this new lock, the lock ordering rules will be
for virNWFilter{Define,Undefine}

       1. nwfilter driver lock
       2. nwfilter update lock


Insert: 3. nwfilter callback drivers lock

This is then used in this order also by nwfilterStateReload


       3. virt driver lock
       4. domain object lock
       5. gentech driver lock

and VM start

       1. nwfilter update lock
       2. virt driver lock
       3. domain object lock
       4. gentech driver lock

Signed-off-by: Daniel P. Berrange <berrange redhat com>
---
  src/nwfilter/nwfilter_driver.c         |  4 +++-
  src/nwfilter/nwfilter_gentech_driver.c | 19 +++++++++++++++++--
  src/nwfilter/nwfilter_gentech_driver.h |  2 +-
  3 files changed, 21 insertions(+), 4 deletions(-)

diff --git a/src/nwfilter/nwfilter_gentech_driver.c b/src/nwfilter/nwfilter_gentech_driver.c
index 89913cf8..d500963 100644
--- a/src/nwfilter/nwfilter_gentech_driver.c
+++ b/src/nwfilter/nwfilter_gentech_driver.c
@@ -936,6 +943,7 @@ _virNWFilterInstantiateFilter(virNWFilterDriverStatePtr driver,
      int rc;

      virNWFilterLockFilterUpdates();
+    virMutexLock(&updateMutex);


Since the filter updates lock had the two purposes before, you are now introducing a separate lock to assign a purpose to each lock.
Further below you are preventing concurrent teardowns to this here.

I am wondering how much further down this lock here could actually be pushed. This and the other function (virNWFilterInstantiateFilterLate) where you place this lock are calling __virNWFilterInstantiateFilter and nothing else calls that function [and the filter read protection above the lock call will remain]. So I think this lock could be placed inside __virNWFilterInstantiateFilter(). Also looking at that function I am not sure whether there is anything worth protecting using this newly introduced lock then. It ends up calling virNWFilterInstantiate(). Here I would be a bit careful with the threads being started to learn the IP addresses. So maybe this function could be the place where to serialize access. What's your take?


   Stefan


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