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

[Fam Zheng <famz@redhat.com>]

On Mon, 12/07 17:42, Cornelia Huck wrote:
> On Mon, 7 Dec 2015 11:02:51 +0100
> Cornelia Huck <cornelia.huck@de.ibm.com> wrote:
> 
> > On Thu,  3 Dec 2015 13:00:00 +0800
> > Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > 
> > > From: Fam Zheng <famz@redhat.com>
> > > 
> > > 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 <famz@redhat.com>
> > > Message-id: 1448962590-2842-2-git-send-email-famz@redhat.com
> > > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > > ---
> > >  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
> > #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 (blk=blk@entry=0x3ffb17e7960)
> >     at /data/git/yyy/qemu/block/block-backend.c:991
> > #3  0x0000000080216d12 in blk_check_byte_request (blk=blk@entry=0x3ffb17e7960, 
> >     offset=offset@entry=4928966656, size=16384)
> >     at /data/git/yyy/qemu/block/block-backend.c:558
> > #4  0x0000000080216df2 in blk_check_request (blk=blk@entry=0x3ffb17e7960, 
> >     sector_num=sector_num@entry=9626888, nb_sectors=nb_sectors@entry=32)
> >     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

I can't parse these lines in the backtrace, what is #8 -> #7 ?

But other than that, this looks like a normal read request. Can you dump "blk"
fields?

> > #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?

It looks like a race, any other operations happening at the time?  What jobs
are in the guest?

It would be nice if you can advise the reproducing approach? Apparently my
attempt failed..

Fam

> 
> Anything further I can do? I guess this patch fixes a bug for someone,
> but it means insta-death for my setup...
>