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Re: [libvirt] libvirtd deadlock on shutdown



Daniel P. Berrange wrote:
> On Wed, Jun 20, 2012 at 03:07:16PM -0600, Jim Fehlig wrote:
>   
>> I'm looking into a libvirtd deadlock on daemon shutdown.  The deadlock
>> occurs when shutting down virNetServer.  If the handling of a job is in
>> flight in virNetServerHandleJob(), the virNetServer lock is acquired
>> when freeing job->prog (src/rpc/virnetserver.c:167).  But the lock is
>> already held in virNetServerFree(), which is blocked in
>> virThreadPoolFree() waiting for all the workers to finish.  No progress
>> can be made.
>>
>> The attached hack fixes the problem, but I'm not convinced this is an
>> appropriate fix.  Is it necessary to hold the virNetServer lock when
>> calling virNetServerProgramFree(job->prog)?  I notice the lock is not
>> held in the error path of virNetServerHandleJob().
>>
>> Thanks,
>> Jim
>>
>>     
>
>   
>> diff --git a/src/rpc/virnetserver.c b/src/rpc/virnetserver.c
>> index ae19e84..edd3196 100644
>> --- a/src/rpc/virnetserver.c
>> +++ b/src/rpc/virnetserver.c
>> @@ -774,7 +774,9 @@ void virNetServerFree(virNetServerPtr srv)
>>      for (i = 0 ; i < srv->nservices ; i++)
>>          virNetServerServiceToggle(srv->services[i], false);
>>  
>> +    virNetServerUnlock(srv);
>>      virThreadPoolFree(srv->workers);
>> +    virNetServerLock(srv);
>>  
>>      for (i = 0 ; i < srv->nsignals ; i++) {
>>          sigaction(srv->signals[i]->signum, &srv->signals[i]->oldaction, NULL);
>>     
>
> The virNetServerPtr object is ref-counted, so technical;ly
> only decrementing the ref count needs to be protected. Once
> the ref count hits 0, then only the current thread (and the
> workers) should be using the virNetServerPtr instance.
>   

Ah, right.  Thanks for clarifying.

> So, yes, it is safe to call virNetServerUnlock before
> virThreadPoolFree. Furthermore, it is /not/ required
> to call virNetServerLock afterwards - no other thread
> should be using it now the workers are dead. So you
> can skip that extra lock call, and also remove the
> unlock call much later
>   

Something like the attached patch?

Regards,
Jim

>From 583be33213e922899b23f036494886397b2549dc Mon Sep 17 00:00:00 2001
From: Jim Fehlig <jfehlig suse com>
Date: Thu, 21 Jun 2012 09:21:44 -0600
Subject: [PATCH] Fix deadlock on libvirtd shutdown

When shutting down libvirtd, the virNetServer shutdown can deadlock
if there are in-flight jobs being handled by virNetServerHandleJob().
virNetServerFree() will acquire the virNetServer lock and call
virThreadPoolFree() to terminate the workers, waiting for the workers
to finish.  But in-flight workers will attempt to acquire the
virNetServer lock, resulting in deadlock.

Fix the deadlock by unlocking the virNetServer lock before calling
virThreadPoolFree().  This is safe since the virNetServerPtr object
is ref-counted and only decrementing the ref count needs to be
protected.  Additionally, there is no need to re-acquire the lock
after virThreadPoolFree() completes as all the workers have
terminated.
---
 src/rpc/virnetserver.c |    6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/src/rpc/virnetserver.c b/src/rpc/virnetserver.c
index ae19e84..9d71e53 100644
--- a/src/rpc/virnetserver.c
+++ b/src/rpc/virnetserver.c
@@ -766,10 +766,9 @@ void virNetServerFree(virNetServerPtr srv)
     virNetServerLock(srv);
     VIR_DEBUG("srv=%p refs=%d", srv, srv->refs);
     srv->refs--;
-    if (srv->refs > 0) {
-        virNetServerUnlock(srv);
+    virNetServerUnlock(srv);
+    if (srv->refs > 0)
         return;
-    }
 
     for (i = 0 ; i < srv->nservices ; i++)
         virNetServerServiceToggle(srv->services[i], false);
@@ -805,7 +804,6 @@ void virNetServerFree(virNetServerPtr srv)
     virNetServerMDNSFree(srv->mdns);
 #endif
 
-    virNetServerUnlock(srv);
     virMutexDestroy(&srv->lock);
     VIR_FREE(srv);
 }
-- 
1.7.9.2


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