Re: [libvirt] [PATCH] nwfilter: Fix the test for the result of atomic dec and test

On Tue, Mar 18, 2014 at 09:45:14PM -0400, Stefan Berger wrote: > From: Stefan Berger <stefanb(a)linux.vnet.ibm.com&gt; > > https://bugzilla.redhat.com/show_bug.cgi?id=1071181 > > Commit 49b59a15 fixed one problem but masks another one related to

> freeing. > > Use virAtomicIntGet() to test for 0 rather than trying to test for

> after virAtomicIntDecAndTest(). > > Avoid putting of the virNWFilterSnoopReq once the thread has been

> > Signed-off-by: Stefan Berger <stefanb(a)linux.vnet.ibm.com&gt; > --- > src/nwfilter/nwfilter_dhcpsnoop.c | 11 +++++++++-- > 1 file changed, 9 insertions(+), 2 deletions(-) > > diff --git a/src/nwfilter/nwfilter_dhcpsnoop.c b/src/nwfilter/ nwfilter_dhcpsnoop.c > index d2a8062..32ca304 100644 > --- a/src/nwfilter/nwfilter_dhcpsnoop.c > +++ b/src/nwfilter/nwfilter_dhcpsnoop.c > @@ -720,7 +720,10 @@ virNWFilterSnoopReqPut(virNWFilterSnoopReqPtr

> > virNWFilterSnoopLock(); > > - if (virAtomicIntDecAndTest(&req->refctr)) { > + virAtomicIntDecAndTest(&req->refctr); > + > + /* make sure it's 0; virAtomitIntDecAndTest may return true on

> + if (virAtomicIntGet(&req->refctr) == 0) { NACK, using two atomic functions, you are making this non-atomic. between these two atomic operations many things can happen an it's not what you want I bet. The virAtomicIntDecAndTest() uses __sync_fetch_and_sub(atomic, 1) and compares it to 1, that's true, but since it is fetch_and_sub (and not the other way around, the value being compared to 1 is the value that the atomic had before it was decremented. That means it returns 1 (true) if and only if the current value is 0. virAtomicIntDecAndTest() is what you really want and should use here.