> From: Vladimir Sementsov-Ogievskiy [mailto:vsement...@virtuozzo.com] > 20.02.2020 11:31, dovgaluk wrote: > > Vladimir Sementsov-Ogievskiy писал 2020-02-19 19:07: > >> 19.02.2020 17:32, dovgaluk wrote: > >>> I encountered a problem with record/replay of QEMU execution and figured > >>> out the > following, when > >>> QEMU is started with one virtual disk connected to the qcow2 image with > >>> applied 'snapshot' > option. > >>> > >>> The patch d710cf575ad5fb3ab329204620de45bfe50caa53 "block/qcow2: > >>> introduce parallel > subrequest handling in read and write" > >>> introduces some kind of race condition, which causes difference in the > >>> data read from the > disk. > >>> > >>> I detected this by adding the following code, which logs IO operation > >>> checksum. And this > checksum may be different in different runs of the same recorded execution. > >>> > >>> logging in blk_aio_complete function: > >>> qemu_log("%"PRId64": blk_aio_complete\n", > >>> replay_get_current_icount()); > >>> QEMUIOVector *qiov = acb->rwco.iobuf; > >>> if (qiov && qiov->iov) { > >>> size_t i, j; > >>> uint64_t sum = 0; > >>> int count = 0; > >>> for (i = 0 ; i < qiov->niov ; ++i) { > >>> for (j = 0 ; j < qiov->iov[i].iov_len ; ++j) { > >>> sum += ((uint8_t*)qiov->iov[i].iov_base)[j]; > >>> ++count; > >>> } > >>> } > >>> qemu_log("--- iobuf offset %"PRIx64" len %x sum: > >>> %"PRIx64"\n", acb- > >rwco.offset, count, sum); > >>> } > >>> > >>> I tried to get rid of aio task by patching qcow2_co_preadv_part: > >>> ret = qcow2_co_preadv_task(bs, ret, cluster_offset, offset, cur_bytes, > >>> qiov, qiov_offset); > >>> > >>> That change fixed a bug, but I have no idea what to debug next to figure > >>> out the exact > reason of the failure. > >>> > >>> Do you have any ideas or hints? > >>> > >> > >> Hi! > >> > >> Hmm, do mean that read from the disk may return wrong data? It would > >> be very bad of course :( > >> Could you provide a reproducer, so that I can look at it and debug? > > > > It is just a winxp-32 image. I record the execution and replay it with the > > following command > lines: > > > > qemu-system-i386 -icount shift=7,rr=record,rrfile=replay.bin -m 512M -drive > file=xp.qcow2,if=none,id=device-34-file,snapshot -drive > driver=blkreplay,if=none,image=device- > 34-file,id=device-34-driver -device > ide-hd,drive=device-34-driver,bus=ide.0,id=device-34 -net > none > > > > qemu-system-i386 -icount shift=7,rr=replay,rrfile=replay.bin -m 512M -drive > file=xp.qcow2,if=none,id=device-34-file,snapshot -drive > driver=blkreplay,if=none,image=device- > 34-file,id=device-34-driver -device > ide-hd,drive=device-34-driver,bus=ide.0,id=device-34 -net > none > > > > Replay stalls at some moment due to the non-determinism of the execution > > (probably caused by > the wrong data read). > > Hmm.. I tried it (with x86_64 qemu and centos image). I waited for some time > for a first > command, than Ctrl+C it. After it replay.bin was 4M. Than started the second > command. It > works, not failing, not finishing. Is it bad? What is expected behavior and > what is wrong?
The second command should finish. There is no replay introspection yet (in master), but you can stop qemu with gdb and inspect replay_state.current_icount field. It should increase with every virtual CPU instruction execution. If that counter has stopped, it means that replay hangs. > >> What is exactly the case? May be you have other parallel aio > >> operations to the same region? > > > > As far as I understand, all aio operations, initiated by IDE controller, > > are performed one- > by-one. > > I don't see anything else in the logs. > > > >> Ideas to experiment: > >> > >> 1. change QCOW2_MAX_WORKERS to 1 or to 2, will it help? > > > > 1 or 2 are ok, and 4 or 8 lead to the failures. > > > >> 2. understand what is the case in code: is it read from one or several > >> clusters, is it aligned, > >> what is the type of clusters, is encryption in use, compression? > > > > There is no encryption and I thinks compression is not enabled too. > > Clusters are read from the temporary overlay: > > > > blk_aio_prwv > > blk_aio_read_entry > > bdrv_co_preadv_part complete offset: 26300000 qiov_offset: 1c200 len: 1e00 > > bdrv_co_preadv_part complete offset: 24723e00 qiov_offset: 0 len: 1c200 > > bdrv_co_preadv_part complete offset: c0393e00 qiov_offset: 0 len: 1e000 > > bdrv_co_preadv_part complete offset: c0393e00 qiov_offset: 0 len: 1e000 > > bdrv_co_preadv_part complete offset: c0393e00 qiov_offset: 0 len: 1e000 > > > > > >> 3. understand what kind of data corruption. What we read instead of > >> correct data? Just garbage, or may be zeroes, or what.. > > > > Most bytes are the same, but some are different: > > > > < 00 00 00 00 00 00 00 00 00 00 00 00 00 00 02 00 > > < 46 49 4c 45 30 00 03 00 18 d1 33 02 00 00 00 00 > > < 01 00 01 00 38 00 01 00 68 01 00 00 00 04 00 00 > > < 00 00 00 00 00 00 00 00 04 00 00 00 9d 0e 00 00 > > < 02 00 00 00 00 00 00 00 10 00 00 00 60 00 00 00 > > --- > >> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 04 00 > >> 46 49 4c 45 30 00 03 00 86 78 35 03 00 00 00 00 > >> 01 00 01 00 38 00 01 00 60 01 00 00 00 04 00 00 > >> 00 00 00 00 00 00 00 00 04 00 00 00 a1 0e 00 00 > >> 04 00 00 00 00 00 00 00 10 00 00 00 60 00 00 00 > > > > That is strange. I could think, that it was caused by the bugs in > > deterministic CPU execution, but the first difference in logs > > occur in READ operation (I dump read/write buffers in blk_aio_complete). > > > > Aha, yes, looks strange. > > Then next steps: > > 1. Does problem hit into the same offset every time? Yes, almost the same offset, almost the same phase of the execution. > 2. Do we write to this region before this strange read? No. > 2.1. If yes, we need to check that we read what we write.. You say you dump > buffers > in blk_aio_complete... I think it would be more reliable to dump at start of > bdrv_co_pwritev and at end of bdrv_co_preadv. Also, guest may modify its > buffers > during operation which would be strange but possible. I dumped every write in file-posix.c handle_aiocb_rw_linear and qemu_pwritev and found no difference in executions. > 2.2 If not, hmm... Exactly. Pavel Dovgalyuk
