[libvirt] [PATCH] qemu_agent: fix deadlock in qemuProcessHandleAgentEOF

[John Ferlan]

On 10/02/2015 08:17 AM, John Ferlan wrote:
> 
> 
> On 09/26/2015 08:18 AM, Wang Yufei wrote:
>> We shutdown a VM A by qemu agent,meanwhile an agent EOF
>> of VM A happened, there's a chance that deadlock occurred:
>>
>> qemuProcessHandleAgentEOF in main thread
>> A)  priv->agent = NULL; //A happened before B
>>
>>     //deadlock when we get agent lock which's held by worker thread
>>     qemuAgentClose(agent);
>>
>> qemuDomainObjExitAgent called by qemuDomainShutdownFlags in worker thread
>> B)  hasRefs = virObjectUnref(priv->agent); //priv->agent is NULL, return false
>>
>>     if (hasRefs)
>>         virObjectUnlock(priv->agent); //agent lock will not be released here
>>
>> So I close agent first, then set priv->agent NULL to fix the deadlock.
>>
>> Signed-off-by: Wang Yufei <james.wangyufei at huawei.com>
>> Reviewed-by: Ren Guannan <renguannan at huawei.com>
>> ---
>>  src/qemu/qemu_process.c | 3 +--
>>  1 file changed, 1 insertion(+), 2 deletions(-)
>>
> 
> Interesting - this is the exact opposite of commit id '1020a504' from
> Michal over 3 years ago.
> 
> However, a bit of digging into the claim from the commit message drove
> me to commit id '362d04779c' which removes the domain object lock that
> was the basis of the initial patch.
> 
> While I'm not an expert in the vmagent code, I do note that the
> HandleAgentEOF code checks for !priv->agent and priv->beingDestroyed,
> while the ExitAgent code doesn't necessarily (or directly) check whether
> the priv->agent is still valid (IOW: that nothing else has removed it
> already like the EOF).
> 
> So, while I don't discount that the patch works - I'm wondering whether
> the smarts/logic should be built into ExitAgent to check for
> !priv->agent (and or something else) that would indicate we're already
> on the path of shutdown.
> 
> John
> 
> 

Ironically after spinning a few cycles and generating another patch, it
seems my initial instincts were correct with respect to commit
362d04779c removing the reason/cause for 1020a504 and thus moving the
qemuAgentClose is the correct option.

Of course I'm not sure what caused me to doubt my first thoughts and
start down the path of trying different ways to resolve this. I think I
somehow got it stuck in my head that AgentDestroy still was taking out a
lock. Oh well, I did learn something at least with respect to how this
code works - so that part is good.  I can also answer my own question
with respect whether ExitAgent needs to check priv->agent and/or
priv->beingDestroyed.

With this patch, the EOF code will take out the vm-lock, then attempt to
take out the agent-lock, but will be 'stuck' waiting for the AgentExit
code to run. Once ExitAgent is called, it will remove it's reference and
unlock. I don't believe there's a way for the !hasRefs to be true, so I
suppose it could be removed...

I'll hold off on pushing to allow pkrempa to make any comments.

John


>> diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
>> index f2586a1..8c9622e 100644
>> --- a/src/qemu/qemu_process.c
>> +++ b/src/qemu/qemu_process.c
>> @@ -150,11 +150,10 @@ qemuProcessHandleAgentEOF(qemuAgentPtr agent,
>>          goto unlock;
>>      }
>>  
>> +    qemuAgentClose(agent);
>>      priv->agent = NULL;
>>  
>>      virObjectUnlock(vm);
>> -
>> -    qemuAgentClose(agent);
>>      return;
>>  
>>   unlock:
>>
> 
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