On 2011-03-08 23:53, Peter Lieven wrote: > Hi, > > during testing of qemu-kvm-0.14.0 i can reproduce the following segfault. i have seen similar crash already in 0.13.0, but had no time to debug. > my guess is that this segfault is related to the threaded vnc server which was introduced in qemu 0.13.0. the bug is only triggerable if a vnc > client is attached. it might also be connected to a resolution change in the guest. i have a backtrace attached. the debugger is still running if someone > needs more output > ... > Thread 1 (Thread 0x7ffff7ff0700 (LWP 29038)): > #0 0x0000000000000000 in ?? () > No symbol table info available. > #1 0x000000000041d669 in main_loop_wait (nonblocking=0) > at /usr/src/qemu-kvm-0.14.0/vl.c:1388 So we are calling a IOHandlerRecord::fd_write handler that is NULL. Looking at qemu_set_fd_handler2, this may happen if that function is called for an existing io-handler entry with non-NULL write handler, passing a NULL write and a non-NULL read handler. And all this without the global mutex held. And there are actually calls in vnc_client_write_plain and vnc_client_write_locked (in contrast to vnc_write) that may generate this pattern. It's probably worth validating that the iothread lock is always held when qemu_set_fd_handler2 is invoked to confirm this race theory, adding something like assert(pthread_mutex_trylock(&qemu_mutex) != 0); (that's for qemu-kvm only) BTW, qemu with just --enable-vnc-thread, ie. without io-thread support, should always run into this race as it then definitely lacks a global mutex. Jan
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