Re: [Qemu-devel] [PATCH 01/10] block: Change BDS parameter of bdrv_open() to **

[Kevin Wolf]

Am 26.01.2014 um 20:02 hat Max Reitz geschrieben:
> Make bdrv_open() take a pointer to a BDS pointer, similarly to
> bdrv_file_open(). If a pointer to a NULL pointer is given, bdrv_open()
> will create a new BDS with an empty name; if the BDS pointer is not
> NULL, that existing BDS will be reused (in the same way as bdrv_open()
> already did).
> 
> Signed-off-by: Max Reitz <address@hidden>
> ---
>  block.c               | 53 
> ++++++++++++++++++++++++++-------------------------
>  block/qcow2.c         | 14 +++++++++-----
>  block/vmdk.c          |  5 ++---
>  block/vvfat.c         |  6 ++----
>  blockdev.c            | 21 ++++++++------------
>  hw/block/xen_disk.c   |  2 +-
>  include/block/block.h |  2 +-
>  qemu-img.c            |  6 +++---
>  qemu-io.c             |  2 +-
>  qemu-nbd.c            |  2 +-
>  10 files changed, 55 insertions(+), 58 deletions(-)
> 
> diff --git a/block.c b/block.c
> index cb21a5f..c660609 100644
> --- a/block.c
> +++ b/block.c
> @@ -1039,7 +1039,7 @@ int bdrv_file_open(BlockDriverState **pbs, const char 
> *filename,
>      }
>  
>      if (!drv->bdrv_file_open) {
> -        ret = bdrv_open(bs, filename, options, flags, drv, &local_err);
> +        ret = bdrv_open(&bs, filename, options, flags, drv, &local_err);
>          options = NULL;
>      } else {
>          ret = bdrv_open_common(bs, NULL, options, flags, drv, &local_err);

In fact, we could use bdrv_new() only in the bdrv_open_common() case now
and leave it to bdrv_open() in the other one. Not sure if it makes
sense, though, I'll have to read the other patches first.

> @@ -1108,8 +1108,6 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict 
> *options, Error **errp)
>                                         sizeof(backing_filename));
>      }
>  
> -    bs->backing_hd = bdrv_new("");
> -
>      if (bs->backing_format[0] != '\0') {
>          back_drv = bdrv_find_format(bs->backing_format);
>      }
> @@ -1118,11 +1116,10 @@ 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);
>  
> -    ret = bdrv_open(bs->backing_hd,
> +    ret = bdrv_open(&bs->backing_hd,
>                      *backing_filename ? backing_filename : NULL, options,
>                      back_flags, back_drv, &local_err);
>      if (ret < 0) {
> -        bdrv_unref(bs->backing_hd);
>          bs->backing_hd = NULL;
>          bs->open_flags |= BDRV_O_NO_BACKING;
>          error_setg(errp, "Could not open backing file: %s",

This one made me think and I find it a bit worrying. I need to review
what the old value of bs->backing_hd is in order to find out if this was
a NULL argument that is already cleaned up or whether it needs to be
cleaned up here (this one is a NULL argument, so this code is fine).

In itself the behaviour is pretty consistent, you only have to free what
you explicity allocated previously. But I'm afraid that ownership of the
reference, especially in the failure case could confuse people.

Perhaps we should add an assert(bs->backing_hd == NULL) before such
bdrv_open() calls? It's not perfect, but maybe better than nothing.

> @@ -1189,8 +1186,6 @@ int bdrv_open_image(BlockDriverState **pbs, const char 
> *filename,
>          /* If a filename is given and the block driver should be detected
>             automatically (instead of using none), use bdrv_open() in order 
> to do
>             that auto-detection. */
> -        BlockDriverState *bs;
 -
>          if (reference) {
>              error_setg(errp, "Cannot reference an existing block device 
> while "
>                         "giving a filename");
> @@ -1198,13 +1193,8 @@ int bdrv_open_image(BlockDriverState **pbs, const char 
> *filename,
>              goto done;
>          }
>  
> -        bs = bdrv_new("");
> -        ret = bdrv_open(bs, filename, image_options, flags, NULL, errp);
> -        if (ret < 0) {
> -            bdrv_unref(bs);
> -        } else {
> -            *pbs = bs;
> -        }
> +        *pbs = NULL;
> +        ret = bdrv_open(pbs, filename, image_options, flags, NULL, errp);
>      } else {
>          ret = bdrv_file_open(pbs, filename, reference, image_options, flags,
>                               errp);
> @@ -1222,14 +1212,17 @@ done:
>   * empty set of options. The reference to the QDict belongs to the block 
> layer
>   * after the call (even on failure), so if the caller intends to reuse the
>   * dictionary, it needs to use QINCREF() before calling bdrv_open.
> + *
> + * If *pbs is NULL, a new BDS will be created with a pointer to it stored 
> there.
> + * If it is not NULL, the referenced BDS will be reused.
>   */
> -int bdrv_open(BlockDriverState *bs, const char *filename, QDict *options,
> +int bdrv_open(BlockDriverState **pbs, const char *filename, QDict *options,
>                int flags, BlockDriver *drv, Error **errp)
>  {
>      int ret;
>      /* TODO: extra byte is a hack to ensure MAX_PATH space on Windows. */
>      char tmp_filename[PATH_MAX + 1];
> -    BlockDriverState *file = NULL;
> +    BlockDriverState *file = NULL, *bs = NULL;

Why do you initialise bs here? It's not supposed to be used before the
first assignment, so let the compiler warn against it.

>      const char *drvname;
>      Error *local_err = NULL;
>  
> @@ -1238,12 +1231,17 @@ int bdrv_open(BlockDriverState *bs, const char 
> *filename, QDict *options,
>          options = qdict_new();
>      }
>  
> +    if (*pbs) {
> +        bs = *pbs;
> +    } else {
> +        bs = bdrv_new("");
> +    }

I think I'd prefer if you moved this above qdict_new(), so that all
option related code stays in one place.

>      bs->options = options;
>      options = qdict_clone_shallow(options);
>  
>      /* For snapshot=on, create a temporary qcow2 overlay */
>      if (flags & BDRV_O_SNAPSHOT) {
> -        BlockDriverState *bs1;
> +        BlockDriverState *bs1 = NULL;
>          int64_t total_size;
>          BlockDriver *bdrv_qcow2;
>          QEMUOptionParameter *create_options;
> @@ -1253,12 +1251,10 @@ int bdrv_open(BlockDriverState *bs, const char 
> *filename, QDict *options,
>             instead of opening 'filename' directly */
>  
>          /* Get the required size from the image */
> -        bs1 = bdrv_new("");
>          QINCREF(options);
> -        ret = bdrv_open(bs1, filename, options, BDRV_O_NO_BACKING,
> +        ret = bdrv_open(&bs1, filename, options, BDRV_O_NO_BACKING,
>                          drv, &local_err);
>          if (ret < 0) {
> -            bdrv_unref(bs1);
>              goto fail;
>          }
>          total_size = bdrv_getlength(bs1) & BDRV_SECTOR_MASK;
> @@ -1386,6 +1382,7 @@ int bdrv_open(BlockDriverState *bs, const char 
> *filename, QDict *options,
>          bdrv_dev_change_media_cb(bs, true);
>      }
>  
> +    *pbs = bs;
>      return 0;
>  
>  unlink_and_fail:
> @@ -1399,13 +1396,20 @@ fail:
>      QDECREF(bs->options);
>      QDECREF(options);
>      bs->options = NULL;
> +    if (!*pbs) {
> +        bdrv_unref(bs);
> +    }
>      if (error_is_set(&local_err)) {
>          error_propagate(errp, local_err);
>      }
>      return ret;
>  
>  close_and_fail:
> -    bdrv_close(bs);
> +    if (*pbs) {
> +        bdrv_close(bs);
> +    } else {
> +        bdrv_unref(bs);
> +    }
>      QDECREF(options);
>      if (error_is_set(&local_err)) {
>          error_propagate(errp, local_err);
> @@ -5276,7 +5280,7 @@ void bdrv_img_create(const char *filename, const char 
> *fmt,
>      size = get_option_parameter(param, BLOCK_OPT_SIZE);
>      if (size && size->value.n == -1) {
>          if (backing_file && backing_file->value.s) {
> -            BlockDriverState *bs;
> +            BlockDriverState *bs = NULL;
>              uint64_t size;
>              char buf[32];
>              int back_flags;
> @@ -5285,9 +5289,7 @@ void bdrv_img_create(const char *filename, const char 
> *fmt,
>              back_flags =
>                  flags & ~(BDRV_O_RDWR | BDRV_O_SNAPSHOT | BDRV_O_NO_BACKING);
>  
> -            bs = bdrv_new("");
> -
> -            ret = bdrv_open(bs, backing_file->value.s, NULL, back_flags,
> +            ret = bdrv_open(&bs, backing_file->value.s, NULL, back_flags,
>                              backing_drv, &local_err);
>              if (ret < 0) {
>                  error_setg_errno(errp, -ret, "Could not open '%s': %s",
> @@ -5295,7 +5297,6 @@ void bdrv_img_create(const char *filename, const char 
> *fmt,
>                                   error_get_pretty(local_err));
>                  error_free(local_err);
>                  local_err = NULL;
> -                bdrv_unref(bs);
>                  goto out;
>              }
>              bdrv_get_geometry(bs, &size);
> diff --git a/block/qcow2.c b/block/qcow2.c
> index 2da62b8..9a25fbd 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -1541,7 +1541,8 @@ static int qcow2_create2(const char *filename, int64_t 
> total_size,
>          goto out;
>      }
>  
> -    bdrv_close(bs);
> +    bdrv_unref(bs);
> +    bs = NULL;
>  
>      /*
>       * And now open the image and make it consistent first (i.e. increase the
> @@ -1550,7 +1551,7 @@ static int qcow2_create2(const char *filename, int64_t 
> total_size,
>       */
>      BlockDriver* drv = bdrv_find_format("qcow2");
>      assert(drv != NULL);
> -    ret = bdrv_open(bs, filename, NULL,
> +    ret = bdrv_open(&bs, filename, NULL,
>          BDRV_O_RDWR | BDRV_O_CACHE_WB | BDRV_O_NO_FLUSH, drv, &local_err);
>      if (ret < 0) {
>          error_propagate(errp, local_err);
> @@ -1597,10 +1598,11 @@ static int qcow2_create2(const char *filename, 
> int64_t total_size,
>          }
>      }
>  
> -    bdrv_close(bs);
> +    bdrv_unref(bs);
> +    bs = NULL;
>  
>      /* Reopen the image without BDRV_O_NO_FLUSH to flush it before returning 
> */
> -    ret = bdrv_open(bs, filename, NULL,
> +    ret = bdrv_open(&bs, filename, NULL,
>                      BDRV_O_RDWR | BDRV_O_CACHE_WB | BDRV_O_NO_BACKING,
>                      drv, &local_err);
>      if (error_is_set(&local_err)) {
> @@ -1610,7 +1612,9 @@ static int qcow2_create2(const char *filename, int64_t 
> total_size,
>  
>      ret = 0;
>  out:
> -    bdrv_unref(bs);
> +    if (bs) {
> +        bdrv_unref(bs);
> +    }
>      return ret;
>  }
>  
> diff --git a/block/vmdk.c b/block/vmdk.c
> index 99ca60f..0fbf230 100644
> --- a/block/vmdk.c
> +++ b/block/vmdk.c
> @@ -1755,10 +1755,9 @@ static int vmdk_create(const char *filename, 
> QEMUOptionParameter *options,
>          goto exit;
>      }
>      if (backing_file) {
> -        BlockDriverState *bs = bdrv_new("");
> -        ret = bdrv_open(bs, backing_file, NULL, BDRV_O_NO_BACKING, NULL, 
> errp);
> +        BlockDriverState *bs = NULL;
> +        ret = bdrv_open(&bs, backing_file, NULL, BDRV_O_NO_BACKING, NULL, 
> errp);
>          if (ret != 0) {
> -            bdrv_unref(bs);
>              goto exit;
>          }
>          if (strcmp(bs->drv->format_name, "vmdk")) {
> diff --git a/block/vvfat.c b/block/vvfat.c
> index 664941c..ae7bc6f 100644
> --- a/block/vvfat.c
> +++ b/block/vvfat.c
> @@ -2936,15 +2936,13 @@ static int enable_write_target(BDRVVVFATState *s)
>          goto err;
>      }
>  
> -    s->qcow = bdrv_new("");
> -
> -    ret = bdrv_open(s->qcow, s->qcow_filename, NULL,
> +    s->qcow = NULL;
> +    ret = bdrv_open(&s->qcow, s->qcow_filename, NULL,
>              BDRV_O_RDWR | BDRV_O_CACHE_WB | BDRV_O_NO_FLUSH, bdrv_qcow,
>              &local_err);
>      if (ret < 0) {
>          qerror_report_err(local_err);
>          error_free(local_err);
> -        bdrv_unref(s->qcow);
>          goto err;
>      }
>  
> diff --git a/blockdev.c b/blockdev.c
> index 36ceece..42163f8 100644
> --- a/blockdev.c
> +++ b/blockdev.c
> @@ -510,7 +510,7 @@ static DriveInfo *blockdev_init(const char *file, QDict 
> *bs_opts,
>      bdrv_flags |= ro ? 0 : BDRV_O_RDWR;
>  
>      QINCREF(bs_opts);
> -    ret = bdrv_open(dinfo->bdrv, file, bs_opts, bdrv_flags, drv, &error);
> +    ret = bdrv_open(&dinfo->bdrv, file, bs_opts, bdrv_flags, drv, &error);
>  
>      if (ret < 0) {
>          error_setg(errp, "could not open disk image %s: %s",
> @@ -1301,12 +1301,11 @@ static void 
> external_snapshot_prepare(BlkTransactionState *common,
>                    qstring_from_str(snapshot_node_name));
>      }
>  
> -    /* We will manually add the backing_hd field to the bs later */
> -    state->new_bs = bdrv_new("");
>      /* TODO Inherit bs->options or only take explicit options with an
>       * extended QMP command? */
> -    ret = bdrv_open(state->new_bs, new_image_file, options,
> +    ret = bdrv_open(&state->new_bs, new_image_file, options,
>                      flags | BDRV_O_NO_BACKING, drv, &local_err);
> +    /* We will manually add the backing_hd field to the bs later */
>      if (ret != 0) {
>          error_propagate(errp, local_err);
>      }
> @@ -1555,7 +1554,7 @@ static void qmp_bdrv_open_encrypted(BlockDriverState 
> *bs, const char *filename,
>      Error *local_err = NULL;
>      int ret;
>  
> -    ret = bdrv_open(bs, filename, NULL, bdrv_flags, drv, &local_err);
> +    ret = bdrv_open(&bs, filename, NULL, bdrv_flags, drv, &local_err);
>      if (ret < 0) {
>          error_propagate(errp, local_err);
>          return;
> @@ -1906,7 +1905,7 @@ void qmp_drive_backup(const char *device, const char 
> *target,
>                        Error **errp)
>  {
>      BlockDriverState *bs;
> -    BlockDriverState *target_bs;
> +    BlockDriverState *target_bs = NULL;
>      BlockDriverState *source = NULL;
>      BlockDriver *drv = NULL;
>      Error *local_err = NULL;
> @@ -1991,10 +1990,8 @@ void qmp_drive_backup(const char *device, const char 
> *target,
>          return;
>      }
>  
> -    target_bs = bdrv_new("");
> -    ret = bdrv_open(target_bs, target, NULL, flags, drv, &local_err);
> +    ret = bdrv_open(&target_bs, target, NULL, flags, drv, &local_err);
>      if (ret < 0) {
> -        bdrv_unref(target_bs);
>          error_propagate(errp, local_err);
>          return;
>      }

It might improve the readability if you leave the declaration without an
initialiser, but have an explicit target_bs = NULL immediately before
the bdrv_open.

> @@ -2027,7 +2024,7 @@ void qmp_drive_mirror(const char *device, const char 
> *target,
>                        Error **errp)
>  {
>      BlockDriverState *bs;
> -    BlockDriverState *source, *target_bs;
> +    BlockDriverState *source, *target_bs = NULL;
>      BlockDriver *drv = NULL;
>      Error *local_err = NULL;
>      int flags;
> @@ -2135,11 +2132,9 @@ void qmp_drive_mirror(const char *device, const char 
> *target,
>      /* Mirroring takes care of copy-on-write using the source's backing
>       * file.
>       */
> -    target_bs = bdrv_new("");
> -    ret = bdrv_open(target_bs, target, NULL, flags | BDRV_O_NO_BACKING, drv,
> +    ret = bdrv_open(&target_bs, target, NULL, flags | BDRV_O_NO_BACKING, drv,
>                      &local_err);
>      if (ret < 0) {
> -        bdrv_unref(target_bs);
>          error_propagate(errp, local_err);
>          return;
>      }

Same here.

> diff --git a/hw/block/xen_disk.c b/hw/block/xen_disk.c
> index 098f6c6..14e6d0b 100644
> --- a/hw/block/xen_disk.c
> +++ b/hw/block/xen_disk.c
> @@ -813,7 +813,7 @@ static int blk_connect(struct XenDevice *xendev)
>              Error *local_err = NULL;
>              BlockDriver *drv = 
> bdrv_find_whitelisted_format(blkdev->fileproto,
>                                                             readonly);
> -            if (bdrv_open(blkdev->bs,
> +            if (bdrv_open(&blkdev->bs,
>                            blkdev->filename, NULL, qflags, drv, &local_err) 
> != 0)
>              {
>                  xen_be_printf(&blkdev->xendev, 0, "error: %s\n",
> diff --git a/include/block/block.h b/include/block/block.h
> index 963a61f..980869d 100644
> --- a/include/block/block.h
> +++ b/include/block/block.h
> @@ -190,7 +190,7 @@ int bdrv_open_image(BlockDriverState **pbs, const char 
> *filename,
>                      QDict *options, const char *bdref_key, int flags,
>                      bool force_raw, bool allow_none, Error **errp);
>  int bdrv_open_backing_file(BlockDriverState *bs, QDict *options, Error 
> **errp);
> -int bdrv_open(BlockDriverState *bs, const char *filename, QDict *options,
> +int bdrv_open(BlockDriverState **pbs, const char *filename, QDict *options,
>                int flags, BlockDriver *drv, Error **errp);
>  BlockReopenQueue *bdrv_reopen_queue(BlockReopenQueue *bs_queue,
>                                      BlockDriverState *bs, int flags);
> diff --git a/qemu-img.c b/qemu-img.c
> index c989850..c39d486 100644
> --- a/qemu-img.c
> +++ b/qemu-img.c
> @@ -289,7 +289,7 @@ static BlockDriverState *bdrv_new_open(const char 
> *filename,
>          drv = NULL;
>      }
>  
> -    ret = bdrv_open(bs, filename, NULL, flags, drv, &local_err);
> +    ret = bdrv_open(&bs, filename, NULL, flags, drv, &local_err);
>      if (ret < 0) {
>          error_report("Could not open '%s': %s", filename,
>                       error_get_pretty(local_err));
> @@ -2314,7 +2314,7 @@ static int img_rebase(int argc, char **argv)
>  
>          bs_old_backing = bdrv_new("old_backing");
>          bdrv_get_backing_filename(bs, backing_name, sizeof(backing_name));
> -        ret = bdrv_open(bs_old_backing, backing_name, NULL, BDRV_O_FLAGS,
> +        ret = bdrv_open(&bs_old_backing, backing_name, NULL, BDRV_O_FLAGS,
>                          old_backing_drv, &local_err);
>          if (ret) {
>              error_report("Could not open old backing file '%s': %s",
> @@ -2324,7 +2324,7 @@ static int img_rebase(int argc, char **argv)
>          }
>          if (out_baseimg[0]) {
>              bs_new_backing = bdrv_new("new_backing");
> -            ret = bdrv_open(bs_new_backing, out_baseimg, NULL, BDRV_O_FLAGS,
> +            ret = bdrv_open(&bs_new_backing, out_baseimg, NULL, BDRV_O_FLAGS,
>                          new_backing_drv, &local_err);
>              if (ret) {
>                  error_report("Could not open new backing file '%s': %s",
> diff --git a/qemu-io.c b/qemu-io.c
> index d669028..2f06dc6 100644
> --- a/qemu-io.c
> +++ b/qemu-io.c
> @@ -68,7 +68,7 @@ static int openfile(char *name, int flags, int growable, 
> QDict *opts)
>      } else {
>          qemuio_bs = bdrv_new("hda");
>  
> -        if (bdrv_open(qemuio_bs, name, opts, flags, NULL, &local_err) < 0) {
> +        if (bdrv_open(&qemuio_bs, name, opts, flags, NULL, &local_err) < 0) {
>              fprintf(stderr, "%s: can't open device %s: %s\n", progname, name,
>                      error_get_pretty(local_err));
>              error_free(local_err);
> diff --git a/qemu-nbd.c b/qemu-nbd.c
> index 136e8c9..0cf123c 100644
> --- a/qemu-nbd.c
> +++ b/qemu-nbd.c
> @@ -597,7 +597,7 @@ int main(int argc, char **argv)
>  
>      bs = bdrv_new("hda");
>      srcpath = argv[optind];
> -    ret = bdrv_open(bs, srcpath, NULL, flags, drv, &local_err);
> +    ret = bdrv_open(&bs, srcpath, NULL, flags, drv, &local_err);
>      if (ret < 0) {
>          errno = -ret;
>          err(EXIT_FAILURE, "Failed to bdrv_open '%s': %s", argv[optind],

So in summary, it looks technically correct, but I'm not sure about
maintainability. If the problems are only for this intermediate state,
I'm okay with leaving them as they are, but otherwise we should try and
figure out something.

Kevin