RE: Race condition in overlayed qcow2?

[Pavel Dovgalyuk]

> From: Vladimir Sementsov-Ogievskiy [mailto:address@hidden]
> 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