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

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 12/08/2015 01:00 PM, Cornelia Huck wrote:
> On Tue, 8 Dec 2015 10:59:54 +0100
> Kevin Wolf <kwolf@redhat.com> wrote:
> 
>> Am 07.12.2015 um 17:42 hat Cornelia Huck geschrieben:
>>> 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
>>
>> 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 (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
>>>> #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.
> 
> 

Some updates after looking at Cornelias system. It seem to be related to 
the F18 gcc that is still on that test system.

Problem happens when hw/block/virtio-blk.c is compiled
with -O2 and goes away with -O1 and -O0 (I trimmed that down to
-fexpensive-optimizations) 

The system calls virtio_blk_submit_multireq 26 times and then it messes
up the blk pointer:

(gdb) display blk->name
1: blk->name = 0x80894300 "drive-virtio-disk0"
(gdb) next
419	            if (niov + req->qiov.niov > IOV_MAX) {
1: blk->name = 0x80894300 "drive-virtio-disk0"
(gdb) 
424	            if (req->qiov.size / BDRV_SECTOR_SIZE + nb_sectors > max_xfer_len) {
1: blk->name = 0x80894300 "drive-virtio-disk0"
(gdb) 
419	            if (niov + req->qiov.niov > IOV_MAX) {
1: blk->name = 0x80894300 "drive-virtio-disk0"
(gdb) 
429	            if (sector_num + nb_sectors != req->sector_num) {
1: blk->name = 0x80894300 "drive-virtio-disk0"
(gdb) 
434	                submit_requests(blk, mrb, start, num_reqs, niov);
1: blk->name = 0x80894300 "drive-virtio-disk0"
(gdb) 
413	    for (i = 0; i < mrb->num_reqs; i++) {
1: blk->name = 0x8089ae40 ""


uninlining submit_request makes the problem go away, so might be a
gcc bug in Fedora18. I am now going to look at the disassembly to be
sure.

Christian