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(a)huawei.com&gt; Reviewed-by: Ren Guannan <renguannan(a)huawei.com&gt; --- src/qemu/qemu_process.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-)

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:

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(a)huawei.com&gt; >> Reviewed-by: Ren Guannan <renguannan(a)huawei.com&gt; >> --- >> 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.

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(a)huawei.com&gt; >>>> Reviewed-by: Ren Guannan <renguannan(a)huawei.com&gt; >>>> --- >>>> 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.

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(a)huawei.com&gt; >> Reviewed-by: Ren Guannan <renguannan(a)huawei.com&gt; >> --- >> 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: >> > > -- > libvir-list mailing list > libvir-list(a)redhat.com > https://www.redhat.com/mailman/listinfo/libvir-list > -- libvir-list mailing list libvir-list(a)redhat.com https://www.redhat.com/mailman/listinfo/libvir-list