Re: [Qemu-devel] [PATCH 04/10] qcow: add qcow_co_write_compressed

[Pavel Butsykin]

On 27.05.2016 20:45, Stefan Hajnoczi wrote:
> On Sat, May 14, 2016 at 03:45:52PM +0300, Denis V. Lunev wrote:
> > +    qemu_co_mutex_lock(&s->lock);
> > +    cluster_offset = get_cluster_offset(bs, sector_num << 9, 2, out_len, 0, 0);
> > +    qemu_co_mutex_unlock(&s->lock);
> > +    if (cluster_offset == 0) {
> > +        ret = -EIO;
> > +        goto fail;
> > +    }
> > +    cluster_offset &= s->cluster_offset_mask;
> > +
> > +    iov = (struct iovec) {
> > +        .iov_base   = out_buf,
> > +        .iov_len    = out_len,
> > +    };
> > +    qemu_iovec_init_external(&hd_qiov, &iov, 1);
> > +    ret = bdrv_co_pwritev(bs->file->bs, cluster_offset, out_len, &hd_qiov, 0);
>
> Not sure if this has the same race condition as the qcow2 patch.  It
> seems that bdrv_getlength() is used to extend the file on a per-sector
> basis.  That would mean compressed data is not packed inside sectors and
> no read-write-modify race condition exists, but I haven't fully audited
> get_cluster_offset().

The get_cluster_offset() also doesn't allow to do multiple compressed
writes in a single cluster, because this function for all offsets
within the cluster returns the same cluster_offset. So here we just
can't write at offset in the cluster, only at the beginning of the
cluster.

> Stefan