Okay, so I'm trying to figure out what's going on with this QEMU process based on the provided GDB backtrace. Let me go through each step and see if I can understand the problem. First, looking at the thread information: - **Thread 1** is blocked in `__libc_recvfrom` at line 27 of `recvfrom.c`. The function is waiting to receive data from a socket (file descriptor 147). This means that this thread isn't making progress because it's stuck waiting for I/O. That could be a problem if the other threads are waiting on this one or if there's a deadlock. - **Thread 2** is in `qemu_mutex_lock_impl` trying to acquire a mutex (`qemu_global_mutex`). The call is at line 66 of `qemu-thread-posix.c`. Since it's blocked here, it suggests that the mutex is currently held by another thread and this thread is waiting to get it. But which thread holds the mutex? Looking further down, I see: - **Thread 2** (the one trying to lock the mutex) has a backtrace leading up to `kvm_cpu_exec` and then into various QEMU internals (`qemu_kvm_cpu_thread_fn`, etc.). This suggests that this thread is part of the KVM acceleration, handling CPU execution. - The `qemu_global_mutex` is in use by **Thread 1**, perhaps. Because Thread 1 is blocked on a socket read, it might have acquired this mutex earlier and not released it because it's waiting for I/O completion. That would leave the mutex held, causing other threads to wait. So, the deadlock situation here seems to be: - **Thread 1** holds `qemu_global_mutex` and is waiting on `__libc_recvfrom`. Since this is a blocking system call, it can't release the mutex until it returns, which isn't happening right now. - Meanwhile, **Thread 2** needs to acquire `qemu_global_mutex` but can't because Thread 1 still holds it. So, Thread 2 is stuck waiting on the mutex. I remember that QEMU uses a main loop where different parts (like I/O and CPU execution) run in separate threads. If one part blocks indefinitely, it can block other parts from making progress, leading to deadlocks or hangs. Looking at the function `main_loop_wait` in `util/main-loop.c:498`, which is called by the main loop, this is likely where Thread 1 is waiting for events (like socket I/O) and has blocked. Meanwhile, the KVM CPU thread (Thread 2) needs to proceed but can't because it's waiting on a mutex held by Thread 1. Possible solutions or things to check: 1. **Reentrant Mutexes**: Maybe the `qemu_global_mutex` isn't reentrant, so if a thread tries to lock it again while already holding it (which could be happening here), it causes a deadlock. But in this case, Thread 2 is a different thread from Thread 1, so that's not the issue. 2. **Mutex Ownership**: Perhaps the way the mutex is being used isn't correct. If multiple threads are waiting on the same resource (like the socket) while holding mutexes, it can create deadlocks. 3. **I/O Multiplexing**: QEMU might be relying on the main loop to handle I/O events without blocking in a way that allows other threads to proceed. If `__libc_recvfrom` is blocking the main thread (Thread 1), then any operations that need the global mutex (like Thread 2) will hang. 4. **Deadlock Detection**: Maybe adding deadlock detection or using a different synchronization mechanism could help, but I'm not sure how feasible that is in QEMU's current setup. 5. **Alternative Synchronization**: Perhaps replacing the mutex with a condition variable or another structure that allows waiting without blocking indefinitely might help, but again, this depends on how QEMU's codebase works. In summary, it looks like Thread 1 is stuck waiting for socket data and holds a global mutex, while Thread 2 is trying to acquire that same mutex and can't proceed. This creates a deadlock where neither thread can make progress. The problem arises from a deadlock in the QEMU process due to improper synchronization between threads handling I/O and CPU execution. **Step-by-Step Explanation:** 1. **Thread 1 (Main Loop)**: - Blocked in `__libc_recvfrom` waiting for data on socket FD 147. - Holds the `qemu_global_mutex`. - Since it's blocked on a system call, it can't release the mutex. 2. **Thread 2 (KVM CPU Execution)**: - Attempting to acquire `qemu_global_mutex` via `qemu_mutex_lock_impl`. - Blocked because Thread 1 holds the mutex. - Cannot proceed with KVM CPU execution without this mutex. 3. **Deadlock Cause**: - Both threads depend on each other: Thread 2 needs the mutex held by Thread 1, which is blocked waiting for I/O. - Without mutual exclusion and proper signaling, neither thread can release resources to allow progress. 4. **Solution Approach**: - Implement non-blocking I/O or use asynchronous operations to prevent the main loop from blocking on system calls while holding critical sections. - Use condition variables or other synchronization primitives that allow waiting without indefinite blocking of threads. - Review and adjust the usage of `qemu_global_mutex` to ensure it's only held during operations that can't be interrupted, reducing the chance of deadlocks. **Answer:** The QEMU process is experiencing a deadlock due to Thread 1 holding the `qemu_global_mutex` while blocked on an I/O operation, preventing Thread 2 from acquiring the mutex. This creates a situation where neither thread can proceed, leading to a hang. To resolve this, consider implementing non-blocking I/O or reevaluating synchronization mechanisms to prevent such deadlocks.