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

John Ferlan jferlan at redhat.com
Mon Oct 26 15:04:57 UTC 2015

On 10/23/2015 10:17 PM, Wang Yufei wrote:
> On 2015/10/23 23:32, John Ferlan wrote:
>> On 10/13/2015 02:36 AM, Wang Yufei wrote:
>>> On 2015/10/2 20:17, 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
>>>>> 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:
>>>> .
>>> Thank you for your reply.
>>> At first, we should consider about the right logic. In my oppinion, we should call
>>> qemuAgentClose at first, then we set priv->agent NULL, just like the logic in qemuProcessStop.
>>>     if (priv->agent) {
>>>         qemuAgentClose(priv->agent);
>>>         priv->agent = NULL;
>>>         priv->agentError = false;
>>>     }
>>> Base on the right logic, we consider about the right lock which I have shown in my patch.
>>> 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?
>>> The answer is yes, we have to, because in ExitAgent we use agent lock which may be released
>>> by other thread like qemuAgentClose, we have to check the agent first to make sure it's
>>> safe to visit.
>> It doesn't feel right that moving qemuAgentClose to inside where the vm
>> lock is held will fix the issue. Or does the virCondSignal (e.g.
>> pthread_cond_signal or pthread_cond_broadcast) release the mutex that
>> would be held by the thread that did the EnterAgent?
>> So let's go back to your scenario again where A happens before B...  As
>> I see it, the bug is in B which has accesses and unreferences
>> priv->agent *without* first getting the vm lock (like other instances),
>> and assuming that priv->agent could still be valid (or the same) between
>> the time the vm was unlocked in EnterAgent and the point at which we are
>> at in ExitAgent.
>> So the bug would then seem to be in the EnterAgent, run Agent command,
>> ExitAgent sequence.  Since we release the vm lock during EnterAgent,
>> "anything" that could alter the priv->agent setting while the lock is
>> not held needs to be handled.
>> Thus perhaps the fix should be (in all callers)
>> agent = priv->agent;
>> EnterAgent(vm);
>> *Agent* Command
>> ExitAgent(vm, agent)
>> Where in ExitAgent then has:
>>     virObjectLock(obj);
>>     if (priv->agent != agent)
>>         VIR_DEBUG("priv->agent=%p differs from agent=%p"
>>                   priv->agent, agent);
>>         virObjectUnlock(agent);
>>         return;
>>     }
>>     if ((hasRefs = virObjectUnref(priv->agent)))
>>         virObjectUnlock(priv->agent);
>>     VIR_DEBUG("Exited agent (agent=%p vm=%p name=%s hasRefs=%d)",
>>               priv->agent, obj, obj->def->name, hasRefs);
>>     priv->agentStart = 0;
>>     if (!hasRefs)
>>         priv->agent = NULL;
>> Does this seem reasonable?
>> John
>> .
> Let's see the order of lock if we modify it in your way:
> Thread B
> require agent lock (qemuDomainObjEnterAgent)
> require vm lock (qemuDomainObjExitAgent, still hold agent lock)
> Thread A
> require vm lock (qemuProcessHandleAgentEOF)
> require agent lock (qemuAgentClose, still hold vm lock)
> Bomb, dead lock! Am I right? We must keep lock require in the same order.

First off - I've removed the fix you proposed from my environment...

If Thread B has the agent-lock and gets vm-lock, then Thread A cannot
get vm-lock until Thread B releases.

So how/when does Thread A get to agent lock (qemuAgentClose)? Well once
Thread B is done with it during virDomainObjEndAPI. So where's the
deadlock? By that time, Thread B has also released the agent-lock.

Conversely, if Thread A (EOF) gets the vm-lock, then Thread B cannot get
it until Thread A releases. Once Thread A releases and Thread B gets it,
then it can determine whether the priv->agent is the same (or valid).

In the EOF code you "could" have the agent-lock on entrance, but you
need the vm-lock in order to clear out the agent from the vm. Calling
qemuAgentClose with the agent passed from the caller (either a shutdown
or some sort of catastrophic failure).

So without any changes we currently have the following:

EnterAgent w/ vm-lock
    get agent-lock

ExitAgent w/ agent-lock
    unref/check agent refs
    if there are refs
    get vm-lock
    adjust vm agent related fields

EOF (may have agent-lock)
    acquire vm-lock
    check vm agent fields
    clear vm->priv->agent

I think the current ordering is correct; however, the problem is the
ExitAgent *assumes* priv->agent is valid without getting the vm-lock.
As you pointed out in your commit message, if EOF runs, it clears
priv->agent (with the vm-lock). If we drop back even further, once we
release the vm-lock after EnterAgent, should not 'assume' priv->agent is

Probably easier if I post some patches to continue debate from there.


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