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fe51174f08
If VM A is shutdown a by qemu agent at appoximately the same time
an agent EOF of VM A happened, there's a chance that deadlock may occur:
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
In order to resolve, during EOF close the agent first, then set priv->agent
to NULL to fix the deadlock.
This essentially reverts commit id '1020a504'. It's also of note that commit
id '362d0477' notes a possible/rare deadlock similar to what was seen in
the monitor in commit id '25f582e3'. However, it seems interceding changes
including commit id 'd960d06f' should remove the deadlock issue.
With this change, if EOF is called:
Get VM lock
Check if !priv->agent || priv->beingDestroyed, then unlock VM
Call qemuAgentClose
Unlock VM
When qemuAgentClose is called
Get Agent lock
If Agent->fd open, close it
Unlock Agent
Unref Agent
qemuDomainObjEnterAgent
Enter with VM lock
Get Agent lock
Increase Agent refcnt
Unlock VM
After running agent command, calling qemuDomainObjExitAgent
Enter with Agent lock
Unref Agent
If not last reference, unlock Agent
Get VM lock
If we were in the middle of an EnterAgent, call Agent command, and
ExitAgent sequence and the EOF code is triggered, then the EOF code
can get the VM lock, make it's checks against !priv->agent ||
priv->beingDestroyed, and call qemuAgentClose. The CloseAgent
would wait to get agent lock. The other thread then will eventually
call ExitAgent, release the Agent lock and unref the Agent. Once
ExitAgent releases the Agent lock, AgentClose will get the Agent
Agent lock, close the fd, unlock the agent, and unref the agent.
The final unref would cause deletion of the agent.
Signed-off-by: Wang Yufei <james.wangyufei@huawei.com>
Reviewed-by: Ren Guannan <renguannan@huawei.com>
LibVirt : simple API for virtualization
Libvirt is a C toolkit to interact with the virtualization capabilities
of recent versions of Linux (and other OSes). It is free software
available under the GNU Lesser General Public License. Virtualization of
the Linux Operating System means the ability to run multiple instances of
Operating Systems concurrently on a single hardware system where the basic
resources are driven by a Linux instance. The library aim at providing
long term stable C API initially for the Xen paravirtualization but
should be able to integrate other virtualization mechanisms if needed.
Daniel Veillard <veillard@redhat.com>