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Some users report (very rarely) seeing a deadlock in the QEMU monitor callbacks Thread 10 (Thread 0x7fcd11e20700 (LWP 26753)): #0 0x00000030d0e0de4d in __lll_lock_wait () from /lib64/libpthread.so.0 #1 0x00000030d0e09ca6 in _L_lock_840 () from /lib64/libpthread.so.0 #2 0x00000030d0e09ba8 in pthread_mutex_lock () from /lib64/libpthread.so.0 #3 0x00007fcd162f416d in virMutexLock (m=<optimized out>) at util/threads-pthread.c:85 #4 0x00007fcd1632c651 in virDomainObjLock (obj=<optimized out>) at conf/domain_conf.c:14256 #5 0x00007fcd0daf05cc in qemuProcessHandleMonitorDestroy (mon=0x7fcccc0029e0, vm=0x7fcccc00a850) at qemu/qemu_process.c:1026 #6 0x00007fcd0db01710 in qemuMonitorDispose (obj=0x7fcccc0029e0) at qemu/qemu_monitor.c:249 #7 0x00007fcd162fd4e3 in virObjectUnref (anyobj=<optimized out>) at util/virobject.c:139 #8 0x00007fcd0db027a9 in qemuMonitorClose (mon=<optimized out>) at qemu/qemu_monitor.c:860 #9 0x00007fcd0daf61ad in qemuProcessStop (driver=driver@entry=0x7fcd04079d50, vm=vm@entry=0x7fcccc00a850, reason=reason@entry=VIR_DOMAIN_SHUTOFF_DESTROYED, flags=flags@entry=0) at qemu/qemu_process.c:4057 #10 0x00007fcd0db323cf in qemuDomainDestroyFlags (dom=<optimized out>, flags=<optimized out>) at qemu/qemu_driver.c:1977 #11 0x00007fcd1637ff51 in virDomainDestroyFlags ( domain=domain@entry=0x7fccf00c1830, flags=1) at libvirt.c:2256 At frame #10 we are holding the domain lock, we call into qemuProcessStop() to cleanup QEMU, which triggers the monitor to close, which invokes qemuProcessHandleMonitorDestroy() which tries to obtain the domain lock again. This is a non-recursive lock, hence hang. Since qemuMonitorPtr is a virObject, the unref call in qemuProcessHandleMonitorDestroy no longer needs mutex protection. The assignment of priv->mon = NULL, can be instead done by the caller of qemuMonitorClose(), thus removing all need for locking. Signed-off-by: Daniel P. Berrange <berrange@redhat.com> (cherry picked from commit 25f582e36a1c066b6c82303b5e4f18eec337a25b)
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>