On Sun, Feb 23, 2014 at 09:54:46AM +0800, Fam Zheng wrote:
> This is the common but non-trivial steps to assign or change the
> backing_hd of BDS.
>
> Signed-off-by: Fam Zheng <f...@redhat.com>
> ---
> block.c | 46 ++++++++++++++++++++++++++++++++++++++--------
> include/block/block.h | 1 +
> 2 files changed, 39 insertions(+), 8 deletions(-)
>
> diff --git a/block.c b/block.c
> index 684b9d6..9caade9 100644
> --- a/block.c
> +++ b/block.c
> @@ -1041,6 +1041,32 @@ fail:
> return ret;
> }
>
> +void bdrv_set_backing_hd(BlockDriverState *bs, BlockDriverState *backing_hd)
> +{
> + if (backing_hd) {
> + /* Grab the reference before unref original backing_hd, so we are
> safe
> + * when rebasing in the backing chain.
> + */
> + bdrv_ref(backing_hd);
> + }
> +
> + if (bs->backing_hd) {
> + bdrv_unref(bs->backing_hd);
> + }
> +
> + bs->backing_hd = backing_hd;
> + if (!backing_hd) {
> + bs->backing_file[0] = '\0';
> + bs->backing_format[0] = '\0';
> + goto out;
> + }
> + pstrcpy(bs->backing_file, sizeof(bs->backing_file),
> backing_hd->filename);
> + pstrcpy(bs->backing_format, sizeof(bs->backing_format),
> + backing_hd->drv ? backing_hd->drv->format_name : "");
> +out:
> + bdrv_refresh_limits(bs);
> +}
> +
> /*
> * Opens the backing file for a BlockDriverState if not yet open
> *
> @@ -1054,6 +1080,7 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict
> *options, Error **errp)
> char backing_filename[PATH_MAX];
> int back_flags, ret;
> BlockDriver *back_drv = NULL;
> + BlockDriverState *backing_hd;
> Error *local_err = NULL;
>
> if (bs->backing_hd != NULL) {
> @@ -1077,6 +1104,8 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict
> *options, Error **errp)
> sizeof(backing_filename));
> }
>
> + backing_hd = bdrv_new("");
> +
> if (bs->backing_format[0] != '\0') {
> back_drv = bdrv_find_format(bs->backing_format);
> }
> @@ -1085,23 +1114,24 @@ int bdrv_open_backing_file(BlockDriverState *bs,
> QDict *options, Error **errp)
> back_flags = bs->open_flags & ~(BDRV_O_RDWR | BDRV_O_SNAPSHOT |
> BDRV_O_COPY_ON_READ);
>
> - assert(bs->backing_hd == NULL);
> - ret = bdrv_open(&bs->backing_hd,
> + ret = bdrv_open(&backing_hd,
> *backing_filename ? backing_filename : NULL, NULL,
> options,
> back_flags, back_drv, &local_err);
> if (ret < 0) {
> bs->backing_hd = NULL;
Assigning bs->backing_hd = NULL is no longer needed now, so this
should be removed.
> + bdrv_unref(backing_hd);
> + backing_hd = NULL;
> bs->open_flags |= BDRV_O_NO_BACKING;
> error_setg(errp, "Could not open backing file: %s",
> error_get_pretty(local_err));
> error_free(local_err);
> return ret;
> }
> -
> - if (bs->backing_hd->file) {
> - pstrcpy(bs->backing_file, sizeof(bs->backing_file),
> - bs->backing_hd->file->filename);
> - }
> + bdrv_set_backing_hd(bs, backing_hd);
> + /* Now we have refcnt = 2 on backing_hd by bdrv_new and
> + * bdrv_set_backing_hd, while we only need 1 */
> + assert(backing_hd->refcnt == 2);
> + bdrv_unref(backing_hd);
It is a bit worrisome to manually track refcnt like this, and I think
it might be red flag that bdrv_set_backing_hd() isn't entirely
correct. It seems wrong (and a bit fragile) to have to compensate for
what is essentially a spurious bdrv_ref() call by another function.
Also, this relies on implicit knowledge that bdrv_set_backing_hd()
increases the refcnt, and that the refcnt is increased only once in
bdrv_set_backing_fd().
>
> /* Recalculate the BlockLimits with the backing file */
> bdrv_refresh_limits(bs);
> @@ -1923,7 +1953,7 @@ void bdrv_append(BlockDriverState *bs_new,
> BlockDriverState *bs_top)
>
> /* The contents of 'tmp' will become bs_top, as we are
> * swapping bs_new and bs_top contents. */
> - bs_top->backing_hd = bs_new;
> + bdrv_set_backing_hd(bs_top, bs_new);
If the bs_top->backing_hd is NULL, this means the refcnt on bs_new is
off by one now, right? And I think in the current usage,
bs_top->backing_hd will always be NULL here.
> bs_top->open_flags &= ~BDRV_O_NO_BACKING;
Hmm, this should probably be in bdrv_set_backing_hd() for any non-NULL
backing_hd value.
> pstrcpy(bs_top->backing_file, sizeof(bs_top->backing_file),
> bs_new->filename);
This line, and the one below it (not in the diff context) are now
redundant, and should be removed.
> diff --git a/include/block/block.h b/include/block/block.h
> index a46f70a..ee1582d 100644
> --- a/include/block/block.h
> +++ b/include/block/block.h
> @@ -208,6 +208,7 @@ int bdrv_parse_discard_flags(const char *mode, int
> *flags);
> int bdrv_open_image(BlockDriverState **pbs, const char *filename,
> QDict *options, const char *bdref_key, int flags,
> bool allow_none, Error **errp);
> +void bdrv_set_backing_hd(BlockDriverState *bs, BlockDriverState *backing_hd);
> int bdrv_open_backing_file(BlockDriverState *bs, QDict *options, Error
> **errp);
> int bdrv_open(BlockDriverState **pbs, const char *filename,
> const char *reference, QDict *options, int flags,
> --
> 1.8.5.4
>
>
