Re: [Qemu-devel] [PATCH] Block: don't do copy-on-read in before_write_notifier

[Wen Congyang]

On 08/19/2015 01:41 PM, Paolo Bonzini wrote:
> On 18/08/2015 19:54, Wen Congyang wrote:
>> We will copy data in before_write_notifier to do backup.
>> It is a nested I/O request, so we cannot do copy-on-read.
> 
> Can you explain why?  What is the bug that this is fixing?

(gdb) bt
#0  0x00007fd53a6cdb55 in raise () from /lib64/libc.so.6
#1  0x00007fd53a6cf131 in abort () from /lib64/libc.so.6
#2  0x00007fd53a6c6a10 in __assert_fail () from /lib64/libc.so.6
#3  0x00007fd53dffe5ad in wait_serialising_requests (self=0x7fd50cdb6ae0) at 
block/io.c:452
#4  0x00007fd53dfff351 in bdrv_aligned_preadv (bs=0x7fd53ea33130, 
req=0x7fd50cdb6ae0, offset=26347307008, bytes=65536, align=512, 
qiov=0x7fd50cdb6c90, flags=
    1) at block/io.c:847
#5  0x00007fd53dfff897 in bdrv_co_do_preadv (bs=0x7fd53ea33130, 
offset=26347307008, bytes=65536, qiov=0x7fd50cdb6c90, 
flags=BDRV_REQ_COPY_ON_READ)
    at block/io.c:970
#6  0x00007fd53dfff962 in bdrv_co_do_readv (bs=0x7fd53ea33130, 
sector_num=51459584, nb_sectors=128, qiov=0x7fd50cdb6c90, flags=0) at 
block/io.c:992
#7  0x00007fd53dfff9cf in bdrv_co_readv (bs=0x7fd53ea33130, 
sector_num=51459584, nb_sectors=128, qiov=0x7fd50cdb6c90) at block/io.c:1001
#8  0x00007fd53ddb077a in backup_do_cow (bs=0x7fd53ea33130, 
sector_num=51459648, nb_sectors=16, error_is_read=0x0) at block/backup.c:132
#9  0x00007fd53ddb0f07 in backup_before_write_notify (notifier=0x7fd5118c9f30, 
opaque=0x7fd50cdb6e40) at block/backup.c:193
#10 0x00007fd53e063193 in notifier_with_return_list_notify 
(list=0x7fd53ea361b8, data=0x7fd50cdb6e40) at util/notify.c:65
#11 0x00007fd53e000079 in bdrv_aligned_pwritev (bs=0x7fd53ea33130, 
req=0x7fd50cdb6e40, offset=26347339776, bytes=8192, qiov=0x7fd54001c848, 
flags=0)
    at block/io.c:1116
#12 0x00007fd53e000b4f in bdrv_co_do_pwritev (bs=0x7fd53ea33130, 
offset=26347339776, bytes=8192, qiov=0x7fd54001c848, flags=0) at block/io.c:1354
#13 0x00007fd53e000c18 in bdrv_co_do_writev (bs=0x7fd53ea33130, 
sector_num=51459648, nb_sectors=16, qiov=0x7fd54001c848, flags=0) at 
block/io.c:1378
#14 0x00007fd53e002dba in bdrv_co_do_rw (opaque=0x7fd53fb76830) at 
block/io.c:2113
#15 0x00007fd53dfafde9 in coroutine_trampoline (i0=1073594560, i1=32725) at 
coroutine-ucontext.c:80
#16 0x00007fd53a6debe0 in __correctly_grouped_prefixwc () from /lib64/libc.so.6
#17 0x0000000000000000 in ?? ()


> 
> Paolo
> 
>> Signed-off-by: Wen Congyang <address@hidden>
>