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When two threads (other than the low-priority TCG VCPU thread) are competing for the iothread lock, a deadlock can happen. This is because iothread_requesting_mutex is set to false by the first thread that gets the mutex, and then the VCPU thread might never yield from the execution loop. If iothread_requesting_mutex is changed from a bool to a counter, the deadlock is fixed. However, there is another bug in qemu_mutex_lock_iothread that can be triggered by the new call_rcu thread. The bug happens if qemu_mutex_lock_iothread is called before the CPUs are created. In that case, first_cpu is NULL and the caller segfaults in qemu_mutex_lock_iothread. To fix this, just do not do the kick if first_cpu is NULL. Reported-by: Leon Alrae <leon.alrae@imgtec.com> Reported-by: Andreas Gustafsson <gson@gson.org> Tested-by: Leon Alrae <leon.alrae@imgtec.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Read the documentation in qemu-doc.html or on http://wiki.qemu-project.org - QEMU team