Re: [Qemu-devel] [PATCH 02/10] qcow2: add qcow2_co_write_compressed

[Kevin Wolf <kwolf@redhat.com>]

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Am 27.05.2016 um 19:33 hat Stefan Hajnoczi geschrieben:
> On Sat, May 14, 2016 at 03:45:50PM +0300, Denis V. Lunev wrote:
> > +    qemu_co_mutex_lock(&s->lock);
> > +    cluster_offset = \
> > +        qcow2_alloc_compressed_cluster_offset(bs, sector_num << 9, out_len);
> 
> The backslash isn't necessary for wrapping lines in C.  This kind of
> thing is only necessary in languages like Python where the grammar is
> whitespace sensistive.
> 
> The C compiler is happy with an arbitrary amount of whitespace
> (newlines) in the middle of a statement.  The backslash in C is handled
> by the preprocessor: it joins the line.  That's useful for macro
> definitions where you need to tell the preprocessor that several lines
> belong to one macro definition.  But it's not needed for normal C code.
> 
> > +    if (!cluster_offset) {
> > +        qemu_co_mutex_unlock(&s->lock);
> > +        ret = -EIO;
> > +        goto fail;
> > +    }
> > +    cluster_offset &= s->cluster_offset_mask;
> >  
> > -        BLKDBG_EVENT(bs->file, BLKDBG_WRITE_COMPRESSED);
> > -        ret = bdrv_pwrite(bs->file->bs, cluster_offset, out_buf, out_len);
> > -        if (ret < 0) {
> > -            goto fail;
> > -        }
> > +    ret = qcow2_pre_write_overlap_check(bs, 0, cluster_offset, out_len);
> > +    qemu_co_mutex_unlock(&s->lock);
> > +    if (ret < 0) {
> > +        goto fail;
> >      }
> >  
> > +    iov = (struct iovec) {
> > +        .iov_base   = out_buf,
> > +        .iov_len    = out_len,
> > +    };
> > +    qemu_iovec_init_external(&hd_qiov, &iov, 1);
> > +
> > +    BLKDBG_EVENT(bs->file, BLKDBG_WRITE_COMPRESSED);
> > +    ret = bdrv_co_pwritev(bs->file->bs, cluster_offset, out_len, &hd_qiov, 0);
> 
> There is a race condition here:
> 
> If the newly allocated cluster is only partially filled by compressed
> data then qcow2_alloc_compressed_cluster_offset() remembers that more
> bytes are still available in the cluster.  The
> qcow2_alloc_compressed_cluster_offset() caller will continue filling the
> same cluster.
> 
> Imagine two compressed writes running at the same time.  Write A
> allocates just a few bytes so write B shares a sector with the first
> write:
> 
>      Sector 1
>   |AAABBBBBBBBB|
> 
> The race condition is that bdrv_co_pwritev() uses read-modify-write (a
> bounce buffer).  If both requests call bdrv_co_pwritev() around the same
> time then the following could happen:
> 
>      Sector 1
>   |000BBBBBBBBB|
> 
> or:
> 
>      Sector 1
>   |AAA000000000|
> 
> It's necessary to hold s->lock around the compressed data write to avoid
> this race condition.

I don't think this race condition exists. bdrv_co_pwritev() can indeed
perform RMW if the lower layer requires alignment, but that's not
something callers need to care about (which would be an awful API) but
is fully handled there by marking requests serialising if they involve
RMW.

Kevin

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