Re: [Qemu-devel] [PULL for-2.5 2/4] block: Don't wait serialising for non-COR read requests

[Cornelia Huck]

On Tue, 8 Dec 2015 10:59:54 +0100
Kevin Wolf <address@hidden> wrote:

> Am 07.12.2015 um 17:42 hat Cornelia Huck geschrieben:
> > On Mon, 7 Dec 2015 11:02:51 +0100
> > Cornelia Huck <address@hidden> wrote:
> > 
> > > On Thu,  3 Dec 2015 13:00:00 +0800
> > > Stefan Hajnoczi <address@hidden> wrote:
> > > 
> > > > From: Fam Zheng <address@hidden>
> > > > 
> > > > The assertion problem was noticed in 06c3916b35a, but it wasn't
> > > > completely fixed, because even though the req is not marked as
> > > > serialising, it still gets serialised by wait_serialising_requests
> > > > against other serialising requests, which could lead to the same
> > > > assertion failure.
> > > > 
> > > > Fix it by even more explicitly skipping the serialising for this
> > > > specific case.
> > > > 
> > > > Signed-off-by: Fam Zheng <address@hidden>
> > > > Message-id: address@hidden
> > > > Signed-off-by: Stefan Hajnoczi <address@hidden>
> > > > ---
> > > >  block/backup.c        |  2 +-
> > > >  block/io.c            | 12 +++++++-----
> > > >  include/block/block.h |  4 ++--
> > > >  trace-events          |  2 +-
> > > >  4 files changed, 11 insertions(+), 9 deletions(-)
> > > 
> > > This one causes segfaults for me:
> > > 
> > > Program received signal SIGSEGV, Segmentation fault.
> > > bdrv_is_inserted (bs=0x800000000000) at /data/git/yyy/qemu/block.c:3071
> > > 3071          if (!drv) {
> > > 
> > > (gdb) bt
> > > #0  bdrv_is_inserted (bs=0x800000000000) at 
> > > /data/git/yyy/qemu/block.c:3071
> 
> This looks like some kind of memory corruption that hit blk->bs. It's
> most definitely not a valid pointer anyway.
> 
> > > #1  0x0000000080216974 in blk_is_inserted (blk=<optimized out>)
> > >     at /data/git/yyy/qemu/block/block-backend.c:986
> > > #2  0x00000000802169c6 in blk_is_available (address@hidden)
> > >     at /data/git/yyy/qemu/block/block-backend.c:991
> > > #3  0x0000000080216d12 in blk_check_byte_request (address@hidden, 
> > >     address@hidden, size=16384)
> > >     at /data/git/yyy/qemu/block/block-backend.c:558
> > > #4  0x0000000080216df2 in blk_check_request (address@hidden, 
> > >     address@hidden, address@hidden)
> > >     at /data/git/yyy/qemu/block/block-backend.c:589
> > > #5  0x0000000080217ee8 in blk_aio_readv (blk=0x3ffb17e7960, sector_num=
> > >     9626888, iov=0x8098c658, nb_sectors=<optimized out>, cb=
> > >     0x80081150 <virtio_blk_rw_complete>, opaque=0x80980620)
> > >     at /data/git/yyy/qemu/block/block-backend.c:727
> > > #6  0x000000008008186e in submit_requests (niov=<optimized out>, 
> > >     num_reqs=<optimized out>, start=<optimized out>, mrb=<optimized out>, 
> > >     blk=<optimized out>) at /data/git/yyy/qemu/hw/block/virtio-blk.c:366
> > > #7  virtio_blk_submit_multireq (mrb=<optimized out>, blk=<optimized out>)
> > >     at /data/git/yyy/qemu/hw/block/virtio-blk.c:444
> > > #8  virtio_blk_submit_multireq (blk=0x3ffb17e7960, mrb=0x3ffffffeb58)
> > >     at /data/git/yyy/qemu/hw/block/virtio-blk.c:389
> > > #9  0x00000000800823ee in virtio_blk_handle_output (vdev=<optimized out>, 
> > >     vq=<optimized out>) at /data/git/yyy/qemu/hw/block/virtio-blk.c:615
> > > #10 0x00000000801e367e in aio_dispatch (ctx=0x80918520)
> > >     at /data/git/yyy/qemu/aio-posix.c:326
> > > #11 0x00000000801d28b0 in aio_ctx_dispatch (source=<optimized out>, 
> > >     callback=<optimized out>, user_data=<optimized out>)
> > >     at /data/git/yyy/qemu/async.c:231
> > > #12 0x000003fffd36a05a in g_main_context_dispatch ()
> > >    from /lib64/libglib-2.0.so.0
> > > #13 0x00000000801e0ffa in glib_pollfds_poll ()
> > >     at /data/git/yyy/qemu/main-loop.c:211
> > > #14 os_host_main_loop_wait (timeout=<optimized out>)
> > >     at /data/git/yyy/qemu/main-loop.c:256
> > > #15 main_loop_wait (nonblocking=<optimized out>)
> > >     at /data/git/yyy/qemu/main-loop.c:504
> > > #16 0x00000000800148a6 in main_loop () at /data/git/yyy/qemu/vl.c:1923
> > > #17 main (argc=<optimized out>, argv=<optimized out>, envp=<optimized 
> > > out>)
> > >     at /data/git/yyy/qemu/vl.c:4684
> > > 
> > > Relevant part of command line:
> > > 
> > > -drive 
> > > file=/dev/sda,if=none,id=drive-virtio-disk0,format=raw,serial=ccwzfcp1,cache=none
> > >  -device 
> > > virtio-blk-ccw,devno=fe.0.0001,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1,scsi=off
> > 
> > I played around a bit. The main part of this change seems to be calling
> > wait_serialising_requests() conditionally; reverting this makes the
> > guest boot again.
> > 
> > I then tried to find out when wait_serialising_requests() was NOT
> > called and added fprintfs: well, it was _always_ called. I then added a
> > fprintf for flags at the beginning of the function: this produced a
> > segfault no matter whether wait_serialising_requests() was called
> > conditionally or unconditionally. Weird race?
> > 
> > Anything further I can do? I guess this patch fixes a bug for someone,
> > but it means insta-death for my setup...
> 
> If it happens immediately, perhaps running under valgrind is possible
> and could give some hints about what happened with blk->bs?

Just a quick update: This triggers on a qemu built with a not-so-fresh
gcc 4.7.2 (and it seems to depend on compiler optimizations as well).
We can't trigger it with newer gccs. No hints from valgrind, sadly.
Investigation ongoing.