Am 07.12.2009 15:16, schrieb Jan Kiszka: >> Likely not. What I did was nothing special, and I did not noticed such a >> crash in the last months. > > And now it happened again (qemu-kvm head, during kernel installation > from network onto local qcow2-disk). Any clever idea how to proceed with > this?
I still haven't seen this and I still have no theory on what could be happening here. I'm just trying to write down what I think must happen to get into this situation. Maybe you can point at something I'm missing or maybe it helps you to have a sudden inspiration. The crash happens because we have a loop in the s->cluster_allocs list. A loop can only be created by inserting an object twice. The only insert to this list happens in qcow2_alloc_cluster_offset (though an earlier call than that of the stack trace). There is only one relevant caller of this function, qcow_aio_write_cb. Part of it is a call to run_dependent_requests which removes the request from s->cluster_allocs. So after the QLIST_REMOVE in run_dependent_requests the request can't be contained in the list, but at the call of qcow2_alloc_cluster_offset it must be contained again. It must be added somewhere in between these two calls. In qcow_aio_write_cb there isn't much happening between these calls. The only thing that could somehow become dangerous is the qcow_aio_write_cb(req, 0); for queued requests in run_dependent_requests. > I could try to run the step in a loop, hopefully retriggering it once in > a (likely longer) while. But then we need some good instrumentation first. I can't explain what exactly would be going wrong there, but if my thoughts are right so far, I think that moving this into a Bottom Half would help. So if you can reproduce it in a loop this could be worth a try. I'd certainly prefer to understand the problem first, but thinking about AIO is the perfect way to make your brain hurt... Kevin
