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Re: [libvirt] Is it a problem that after virEventRegisterDefaultImpl we have handlers leaked



On Tue, Sep 03, 2013 at 10:25:45AM +0000, Wangyufei (A) wrote:
> 
> 
> > -----Original Message-----
> > From: Daniel P. Berrange [mailto:berrange redhat com]
> > Sent: Tuesday, September 03, 2013 5:14 PM
> > To: Wangyufei (A)
> > Cc: libvir-list redhat com; Wangrui (K)
> > Subject: Re: [libvirt] Is it a problem that after virEventRegisterDefaultImpl we
> > have handlers leaked
> > 
> > On Mon, Sep 02, 2013 at 07:44:39AM +0000, Wangyufei (A) wrote:
> > > Hello,
> > > When I ran programme event-test compiled from event-test.c, I found a
> > problem that, after virEventRegisterDefaultImpl I do virConnectOpenAuth
> > and virConnectClose, there will be handlers of socket and pipe opened by
> > virConnectOpenAuth leaked after virConnectClose. So I did some analysis,
> > and I found the fact following:
> > >
> > > In the condition that we only have one connection here
> > >
> > > int virNetSocketAddIOCallback(virNetSocketPtr sock,
> > >                               int events,
> > >                               virNetSocketIOFunc func,
> > >                               void *opaque,
> > >                               virFreeCallback ff)
> > > {
> > >     int ret = -1;
> > >
> > >     virObjectRef(sock); //Here we add sock refers once, then we will get
> > refers equal 2 after
> > >     virObjectLock(sock);
> > >     if (sock->watch > 0) {
> > >         VIR_DEBUG("Watch already registered on socket %p", sock);
> > >         goto cleanup;
> > >     }
> > >
> > >     if ((sock->watch = virEventAddHandle(sock->fd, //If we have called
> > virEventRegisterDefaultImpl, then here we'll get a sock watch non negative
> > and will not go to cleanup.
> > >                                          events,
> > >
> > virNetSocketEventHandle,
> > >                                          sock,
> > >                                          virNetSocketEventFree))
> > < 0) {
> > >         VIR_DEBUG("Failed to register watch on socket %p", sock);
> > >         goto cleanup;
> > >     }
> > >     sock->func = func;
> > >     sock->opaque = opaque;
> > >     sock->ff = ff;
> > >
> > >     ret = 0;
> > >
> > > cleanup:
> > >     virObjectUnlock(sock);
> > >     if (ret != 0)
> > >         virObjectUnref(sock); //If we haven't called
> > virEventRegisterDefaultImpl, we'll be here after virEventAddHandle, and
> > sock refers will decrease to 1
> > >     return ret;
> > > }
> > >
> > > Condition with virEventRegisterDefaultImpl, we'll do unrefer action in two
> > places:
> > >
> > > 1.       virEventRunDefaultImpl  ->virEventPollRunOnce
> > ->virEventRunDefaultImpl  ->virEventPollRunOnce
> > ->virEventPollCleanupHandles -> virNetSocketEventFree -> virObjectUnref
> > >
> > > 2.       virConnectClose ->virObjectUnref ->virConnectDispose
> > ->remoteConnectClose  ->doRemoteClose  ->virNetClientClose
> > ->virNetClientCloseInternal -> virNetClientIOEventLoopPassTheBuck ->
> > virNetClientCloseLocked -> virObjectUnref
> > >
> > > When event dealing loop is alive, everything work fine, we'll get sock
> > refers 2
> > > after virConnectOpenAuth and unrefer twice in two places above after
> > virConnectClose.
> > > But If some accidents happened like we quit event dealing loop or
> > virEventRunDefaultImpl
> > > suspended, then sock refers will never be zero, and handlers will never be
> > freed.
> > 
> > Do not stop running the event loop. It is a requirement of the API that once
> > you have
> > called  virEventRegisterDefaultImpl, you *must* always execute the event
> > loop forever
> > after until your application exits.
> > 
> > 
> > > I consider to add something like virEventDeregisterDefaultImpl to set
> > addHandleImpl and his buddy NULL. Apparently it is far away from fixing it
> > completely.
> > 
> > No, stopping or de-registering event loop impls is a non-goal.
> > 
> > > At last I have two questions here:
> > >
> > >
> > > 1.       Is it a problem that after virEventRegisterDefaultImpl we have
> > handlers leaked?
> > 
> > There are no handlers leaked if you run the event loop, which is a
> > requirement
> > of the API.
> 
> Well, Is there any tiny chance that event loop stopped?
> If that happened now, the handlers leak will affect the whole host OS, maybe no handlers to use at last.
> Is that what we expect?
> Can we do something to reduce the impact even if something impossible happened?

If the event loop has bugs which cause it to stop then those
should be fixed. This isn't something we need / want to work
around elsewhere.

Daniel
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