[Libvir] [PATCH] avoid virsh hang due to missing virDomainFree(dom) call

[Jim Meyering]

"Daniel P. Berrange" <berrange at redhat.com> wrote:

> On Wed, Jan 30, 2008 at 06:25:19PM +0100, Jim Meyering wrote:
>> Without the following patch, this command would hang
>>
>>     printf 'domuuid fc4\ndomstate fc4\n' \
>>       | ./virsh --connect test://$PWD/../docs/testnode.xml
>>
>> with this stack trace:
>>
>>     __lll_lock_wait ...
>>     _L_lock_105 ...
>>     __pthread_mutex_lock ...
>>     virUnrefDomain (domain=0x6a8b30) at hash.c:884
>>     virDomainFree (domain=0x6a8b30) at libvirt.c:1211
>>     cmdDomstate (ctl=0x7fffea723390, cmd=0x6a8b10) at virsh.c:677
>>     vshCommandRun (ctl=0x7fffea723390, cmd=0x6a8b10) at virsh.c:4033
>>     main (argc=3, argv=0x7fffea7234e8) at virsh.c:501
>>
>> The problem is that cmdDomuuid didn't call virDomainFree(dom), so
>> cmdDomstate hangs trying to do the Unref.
>
> I don't understand why it would hang simply because it forgot to free
> an object. Surely it ought to just be a memory leak not hang ? Each
> individual libvirt API must always have a matched lock & unlock pair
> in all codepaths, so a hang should only occur if an unlock is missing
> in some codepath.

Well maybe virDomainFree is misnamed then.
It does more:

The missing virDomainFree calls cause that because
they are what is supposed to do the unlock.

virDomainFree calls virUnrefDomain, which calls virReleaseDomain
(when refs hits 0), which calls pthread_mutex_unlock.