Kevin Wolf wrote: > 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. If m->nb_clusters is not, the entry won't be removed from the list. And of something corrupted nb_clusters so that it became 0 although it's still enqueued, we would see the deadly loop I faced, right? Unfortunately, any arbitrary memory corruption that generates such zeros can cause this... Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
