On 04/06/2017 04:15 PM, Richard W.M. Jones wrote:

>>>> +++ b/block/io.c
>>>> @@ -945,6 +945,8 @@ static int coroutine_fn bdrv_co_do_copy_on_readv(BdrvChild *child,
>>>>      size_t skip_bytes;
>>>>      int ret;
>>>>  
>>>> +    assert(child->perm & (BLK_PERM_WRITE_UNCHANGED | BLK_PERM_WRITE));
>>>

> https://bugzilla.redhat.com/show_bug.cgi?id=1439922
> 
> There is also a minimal reproducer in comment 2.

If it helps, backtrace shows:

#4  0x0000555555c09f10 in bdrv_co_do_copy_on_readv
(child=0x555556990180, offset=0, bytes=512, qiov=0x7fffffffd0c0) at
block/io.c:948
#5  0x0000555555c0a33d in bdrv_aligned_preadv (child=0x555556990180,
req=0x7fffb6dffec0, offset=0, bytes=512, align=1, qiov=0x7fffffffd0c0,
flags=1)
    at block/io.c:1058
#6  0x0000555555c0a8c3 in bdrv_co_preadv (child=0x555556990180,
offset=0, bytes=512, qiov=0x7fffffffd0c0, flags=BDRV_REQ_COPY_ON_READ)
at block/io.c:1166
#7  0x0000555555bf7ccf in blk_co_preadv (blk=0x555556a2bc20, offset=0,
bytes=512, qiov=0x7fffffffd0c0, flags=0) at block/block-backend.c:927
#8  0x0000555555bf7e19 in blk_read_entry (opaque=0x7fffffffd0e0)
    at block/block-backend.c:974
#9  0x0000555555cc3015 in coroutine_trampoline (i0=1485566720, i1=21845)
    at util/coroutine-ucontext.c:79

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
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