Re: [PATCH v2 7/7] block-copy: protect BlockCopyState .method fields

[Paolo Bonzini]

On 26/05/21 19:18, Vladimir Sementsov-Ogievskiy wrote:
> It's actually the original idea of block_copy_do_copy() function: do 
> only simple copy, don't interact with the state. It's a kind of wrapper 
> on top of bdrv_co<io functions>.
>
> So, actually updating s->use_copy_range here was a bad idea.

It's still more complicated, because you need to add some kind of

        method = s->method;
        ret = block_copy_do_copy(..., method);
        if (ret < 0 && method <= COPY_RANGE_SMALL) {
            method = COPY_RANGE_READ_WRITE;
            ret = block_copy_do_copy(..., method);
        }
        lock();
        if (method == s->method) {
            /* compute new method */
        }

which makes it more complicated than this patch IMO.  But yeah at least 
it's a viable alternative to the atomics.

Paolo