Hu Tao wrote:
>> >From 583be33213e922899b23f036494886397b2549dc Mon Sep 17 00:00:00 2001
>> From: Jim Fehlig <jfehlig@xxxxxxxx>
>> 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@xxxxxxxxxxxxxx>
> 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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