Re: [Qemu-block] [Qemu-devel] [PATCH] iotests: Tweak 030 in order to trigger a race condition with parallel jobs

[Alberto Garcia]

On Fri 08 Dec 2017 08:13:48 PM CET, John Snow <address@hidden> wrote:

>>>>              qemu_io('-f', iotests.imgfmt,
>>>> -                    '-c', 'write -P %d %d %d' % (i, i*1024*1024, num_kb * 
>>>> 1024),
>>>> +                    '-c', 'write -P 0xFF %dk %dk' % (i * 512, num_kb),
>>>
>>> I guess changing from a variable to a fixed 0xff pattern doesn't
>>> make a difference?
>> 
>> I noticed that with the previous code we would write zeroes to the
>> first image (i == 0), and with that I can't reproduce the bug. I
>> assume that block-stream doesn't copy the data in that case. Changing
>> it to anything != 0 solves the problem.
>
> I think I ran into a similar problem with an AHCI test once.

It's this bit from bdrv_co_do_copy_on_readv() (which is the way
block-stream is implemented):

    if (drv->bdrv_co_pwrite_zeroes &&
        buffer_is_zero(bounce_buffer, pnum)) {
        /* FIXME: Should we (perhaps conditionally) be setting
         * BDRV_REQ_MAY_UNMAP, if it will allow for a sparser copy
         * that still correctly reads as zero? */
        ret = bdrv_co_do_pwrite_zeroes(bs, cluster_offset, pnum, 0);
    } else {
        /* This does not change the data on the disk, it is not
         * necessary to flush even in cache=writethrough mode.
         */
        ret = bdrv_driver_pwritev(bs, cluster_offset, pnum,
                                  &local_qiov, 0);
    }

Berto