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Re: [libvirt] libvirtd deadlock on shutdown



Hu Tao wrote:
>> >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);
>>     
>
> At this point other threads may have changed srv->refs...
>
>   
>> +    if (srv->refs > 0)
>>     
>
> ...so it's unsafe to test srv->refs here without locking.
>
> For example, assume srv->refs is 2 at the beginning of virNetServerFree,
>
>   thread A              thread B
>
>   lock srv              lock srv (blocked)
>
>   dec srv->refs
>
>   (srv->refs is 1)
>
>   unlock srv
>
>                         lock srv
>
> 			dec srv->refs
>
> 			(srv->refs is 0)
>
> 			unlock src
>
>   test srv->refs        test srv->refs
>
>
> In this case both threads have found srv->refs is 0 and are going to
> free srv...
>   

Hmm, I thought it was dangerous to read srv->refs after unlocking, and
then wrote the code that way anyhow :-/.

> Following patch fixes problem.
>
> >From 25aa97e05aa76054b781a4e5e83781ee16d5afee Mon Sep 17 00:00:00 2001
> From: Hu Tao <hutao cn fujitsu com>
> Date: Fri, 22 Jun 2012 11:15:01 +0800
> Subject: [PATCH] fix a bug of ref count in virnetserver.c
>
> The test of ref count is not protected by lock, which is unsafe because
> the ref count may have been changed by other threads during the test.
>
> This patch fixes this.
>   

ACK,  I've pushed this.

Thanks!
Jim

> ---
>  src/rpc/virnetserver.c |    5 +++--
>  1 files changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/src/rpc/virnetserver.c b/src/rpc/virnetserver.c
> index 9d71e53..247ddd7 100644
> --- a/src/rpc/virnetserver.c
> +++ b/src/rpc/virnetserver.c
> @@ -759,15 +759,16 @@ void virNetServerQuit(virNetServerPtr srv)
>  void virNetServerFree(virNetServerPtr srv)
>  {
>      int i;
> +    int refs;
>  
>      if (!srv)
>          return;
>  
>      virNetServerLock(srv);
>      VIR_DEBUG("srv=%p refs=%d", srv, srv->refs);
> -    srv->refs--;
> +    refs = --srv->refs;
>      virNetServerUnlock(srv);
> -    if (srv->refs > 0)
> +    if (refs > 0)
>          return;
>  
>      for (i = 0 ; i < srv->nservices ; i++)
>   


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