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A deadlock situation can occur when autostarting a LXC domain 'guest'
due to two threads attempting to take opposing locks while holding
opposing locks (AB BA problem). Thread A takes and holds the 'vm' lock
while attempting to take the 'client' lock, meanwhile, thread B takes
and holds the 'client' lock while attempting to take the 'vm' lock.
The potential for this can be seen as follows:
Thread A:
virLXCProcessAutostartDomain (takes vm lock)
--> virLXCProcessStart
--> virLXCProcessConnectMonitor
--> virLXCMonitorNew
--> virNetClientSetCloseCallback (wants client lock)
Thread B:
virNetClientIncomingEvent (takes client lock)
--> virNetClientIOHandleInput
--> virNetClientCallDispatch
--> virNetClientCallDispatchMessage
--> virNetClientProgramDispatch
--> virLXCMonitorHandleEventInit
--> virLXCProcessMonitorInitNotify (wants vm lock)
Since these threads are scheduled independently and are preemptible it
is possible for the deadlock scenario to occur where each thread locks
their first lock but both will fail to get their second lock and just
spin forever. You get something like:
virLXCProcessAutostartDomain (takes vm lock)
--> virLXCProcessStart
--> virLXCProcessConnectMonitor
--> virLXCMonitorNew
<...>
virNetClientIncomingEvent (takes client lock)
--> virNetClientIOHandleInput
--> virNetClientCallDispatch
--> virNetClientCallDispatchMessage
--> virNetClientProgramDispatch
--> virLXCMonitorHandleEventInit
--> virLXCProcessMonitorInitNotify (wants vm lock but spins)
<...>
--> virNetClientSetCloseCallback (wants client lock but spins)
Neither thread ever gets the lock it needs to be able to continue
while holding the lock that the other thread needs.
The actual window for preemption which can cause this deadlock is
rather small, between the calls to virNetClientProgramNew() and
execution of virNetClientSetCloseCallback(), both in
virLXCMonitorNew(). But it can be seen in real world use that this
small window is enough.
By moving the call to virNetClientSetCloseCallback() ahead of
virNetClientProgramNew() we can close any possible chance of the
deadlock taking place. There should be no other implications to the
move since the close callback (in the unlikely event was called) will
spin on the vm lock. The remaining work that takes place between the
old call location of virNetClientSetCloseCallback() and the new
location is unaffected by the move.
Signed-off-by: Mark Asselstine <mark.asselstine@windriver.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
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|---|---|---|
| .. | ||
| access | ||
| admin | ||
| bhyve | ||
| conf | ||
| cpu | ||
| cpu_map | ||
| esx | ||
| hyperv | ||
| interface | ||
| keycodemapdb@16e5b07876 | ||
| libxl | ||
| locking | ||
| logging | ||
| lxc | ||
| network | ||
| node_device | ||
| nwfilter | ||
| openvz | ||
| phyp | ||
| qemu | ||
| remote | ||
| rpc | ||
| secret | ||
| security | ||
| storage | ||
| test | ||
| uml | ||
| util | ||
| vbox | ||
| vmware | ||
| vmx | ||
| vz | ||
| xenapi | ||
| xenconfig | ||
| admin_protocol-structs | ||
| check-aclperms.pl | ||
| check-aclrules.pl | ||
| check-driverimpls.pl | ||
| check-drivername.pl | ||
| check-symfile.pl | ||
| check-symsorting.pl | ||
| datatypes.c | ||
| datatypes.h | ||
| driver-hypervisor.h | ||
| driver-interface.h | ||
| driver-network.h | ||
| driver-nodedev.h | ||
| driver-nwfilter.h | ||
| driver-secret.h | ||
| driver-state.h | ||
| driver-storage.h | ||
| driver-stream.h | ||
| driver.c | ||
| driver.h | ||
| dtrace2systemtap.pl | ||
| internal.h | ||
| libvirt_admin_private.syms | ||
| libvirt_admin_public.syms | ||
| libvirt_atomic.syms | ||
| libvirt_driver_modules.syms | ||
| libvirt_esx.syms | ||
| libvirt_internal.h | ||
| libvirt_libssh2.syms | ||
| libvirt_libssh.syms | ||
| libvirt_linux.syms | ||
| libvirt_lxc.syms | ||
| libvirt_openvz.syms | ||
| libvirt_private.syms | ||
| libvirt_probes.d | ||
| libvirt_public.syms | ||
| libvirt_qemu_probes.d | ||
| libvirt_qemu.syms | ||
| libvirt_remote.syms | ||
| libvirt_sasl.syms | ||
| libvirt_vmware.syms | ||
| libvirt_vmx.syms | ||
| libvirt_xenconfig.syms | ||
| libvirt-admin.c | ||
| libvirt-admin.conf | ||
| libvirt-domain-snapshot.c | ||
| libvirt-domain.c | ||
| libvirt-host.c | ||
| libvirt-interface.c | ||
| libvirt-lxc.c | ||
| libvirt-lxc.pc.in | ||
| libvirt-network.c | ||
| libvirt-nodedev.c | ||
| libvirt-nwfilter.c | ||
| libvirt-qemu.c | ||
| libvirt-qemu.pc.in | ||
| libvirt-secret.c | ||
| libvirt-storage.c | ||
| libvirt-stream.c | ||
| libvirt.c | ||
| libvirt.conf | ||
| libvirt.pc.in | ||
| lock_protocol-structs | ||
| lxc_monitor_protocol-structs | ||
| lxc_protocol-structs | ||
| Makefile.am | ||
| qemu_protocol-structs | ||
| README | ||
| remote_protocol-structs | ||
| virkeepaliveprotocol-structs | ||
| virnetprotocol-structs | ||
libvirt library code README
===========================
The directory provides the bulk of the libvirt codebase. Everything
except for the libvirtd daemon and client tools. The build uses a
large number of libtool convenience libraries - one for each child
directory, and then links them together for the final libvirt.so,
although some bits get linked directly to libvirtd daemon instead.
The files directly in this directory are supporting the public API
entry points & data structures.
There are two core shared modules to be aware of:
* util/ - a collection of shared APIs that can be used by any
code. This directory is always in the include path
for all things built
* conf/ - APIs for parsing / manipulating all the official XML
files used by the public API. This directory is only
in the include path for driver implementation modules
* vmx/ - VMware VMX config handling (used by esx/ and vmware/)
Then there are the hypervisor implementations:
* bhyve - bhyve - The BSD Hypervisor
* esx/ - VMware ESX and GSX support using vSphere API over SOAP
* hyperv/ - Microsoft Hyper-V support using WinRM
* lxc/ - Linux Native Containers
* openvz/ - OpenVZ containers using cli tools
* phyp/ - IBM Power Hypervisor using CLI tools over SSH
* qemu/ - QEMU / KVM using qemu CLI/monitor
* remote/ - Generic libvirt native RPC client
* test/ - A "mock" driver for testing
* uml/ - User Mode Linux
* vbox/ - Virtual Box using native API
* vmware/ - VMware Workstation and Player using the vmrun tool
* xen/ - Xen using hypercalls, XenD SEXPR & XenStore
* xenapi/ - Xen using libxenserver
Finally some secondary drivers that are shared for several HVs.
Currently these are used by LXC, OpenVZ, QEMU, UML and Xen drivers.
The ESX, Hyper-V, Power Hypervisor, Remote, Test & VirtualBox drivers all
implement the secondary drivers directly
* cpu/ - CPU feature management
* interface/ - Host network interface management
* network/ - Virtual NAT networking
* nwfilter/ - Network traffic filtering rules
* node_device/ - Host device enumeration
* secret/ - Secret management
* security/ - Mandatory access control drivers
* storage/ - Storage management drivers
Since both the hypervisor and secondary drivers can be built as
dlopen()able modules, it is *FORBIDDEN* to have build dependencies
between these directories. Drivers are only allowed to depend on
the public API, and the internal APIs in the util/ and conf/
directories