On 2011-03-09 11:16, Peter Lieven wrote: > > Am 09.03.2011 um 08:37 schrieb Jan Kiszka: > >> 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) > > qemu_mutex doesn't exists (anymore). i can only find qemu_global_mutex and qemu_fair_mutex. In qemu-kvm, qemu-kvm.c contains that lock. In upstream, it's qemu_global_mutex, but you will have to enable --enable-io-thread and export it. > > i try with qemu_global_mutex now. if thats not correct please tell. I think we knokw know the call path, so you could also wait for Corentin to send a patch and test that one. Jan
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