Re: [Qemu-devel] [RFC Patch 4/7]Qemu: Framework for reopening image files safely

[Kevin Wolf <kwolf@redhat.com>]

Am 01.02.2012 04:06, schrieb Supriya Kannery:
> Struct BDRVReopenState along with three reopen related functions
> introduced for handling reopening of images safely. This can be
> extended by each of the block drivers to reopen respective
> image files.
> 
> Signed-off-by: Supriya Kannery <supriyak@linux.vnet.ibm.com>
> 
> Index: qemu/block.c
> ===================================================================
> --- qemu.orig/block.c
> +++ qemu/block.c
> @@ -808,10 +808,32 @@ unlink_and_fail:
>      return ret;
>  }
>  
> +int bdrv_reopen_prepare(BlockDriverState *bs, BDRVReopenState **prs, int flags)
> +{
> +     BlockDriver *drv = bs->drv;
> +
> +     return drv->bdrv_reopen_prepare(bs, prs, flags);
> +}
> +
> +void bdrv_reopen_commit(BlockDriverState *bs, BDRVReopenState *rs)
> +{
> +    BlockDriver *drv = bs->drv;
> +
> +    drv->bdrv_reopen_commit(bs, rs);
> +}
> +
> +void bdrv_reopen_abort(BlockDriverState *bs, BDRVReopenState *rs)
> +{
> +    BlockDriver *drv = bs->drv;
> +
> +    drv->bdrv_reopen_abort(bs, rs);
> +}
> +
>  int bdrv_reopen(BlockDriverState *bs, int bdrv_flags)
>  {
>      BlockDriver *drv = bs->drv;
>      int ret = 0, open_flags;
> +    BDRVReopenState *reopen_state = NULL;
>  
>      /* Quiesce IO for the given block device */
>      qemu_aio_flush();
> @@ -820,17 +842,32 @@ int bdrv_reopen(BlockDriverState *bs, in
>          qerror_report(QERR_DATA_SYNC_FAILED, bs->device_name);
>          return ret;
>      }
> -    open_flags = bs->open_flags;
> -    bdrv_close(bs);
>  
> -    ret = bdrv_open(bs, bs->filename, bdrv_flags, drv);
> -    if (ret < 0) {
> -        /* Reopen failed. Try to open with original flags */
> -        qerror_report(QERR_REOPEN_FILE_FAILED, bs->filename);
> -        ret = bdrv_open(bs, bs->filename, open_flags, drv);
> +    /* Use driver specific reopen() if available */
> +    if (drv->bdrv_reopen_prepare) {
> +        ret = bdrv_reopen_prepare(bs, &reopen_state, bdrv_flags);
> +         if (ret < 0) {
> +            bdrv_reopen_abort(bs, reopen_state);
> +            qerror_report(QERR_REOPEN_FILE_FAILED, bs->filename);
> +            return ret;
> +        }
> +
> +        bdrv_reopen_commit(bs, reopen_state);
> +        bs->open_flags = bdrv_flags;
> +
> +    } else {
> +       open_flags = bs->open_flags;
> +       bdrv_close(bs);
> +
> +       ret = bdrv_open(bs, bs->filename, bdrv_flags, drv);
>          if (ret < 0) {
> -            /* Reopen failed with orig and modified flags */
> -            abort();
> +            /* Reopen failed. Try to open with original flags */
> +            qerror_report(QERR_REOPEN_FILE_FAILED, bs->filename);
> +            ret = bdrv_open(bs, bs->filename, open_flags, drv);
> +            if (ret < 0) {
> +                /* Reopen failed with orig and modified flags */
> +                bs->drv = NULL;
> +            }
>          }

Most image formats don't have a bdrv_reopen_* implementation after this
series, so usually you'll have something like qcow2 on top of file. This
code uses bdrv_close/open for the whole stack, even though the file
layer could actually make use of a bdrv_reopen_* implementation and the
qcow2 open isn't likely to fail if the image file could be opened.

I think we can use drv->bdrv_close/open to reopen only one layer and try
using bdrv_reopen_* for the lower layer again.

This is an improvement that can be done in a separate patch, though.

Kevin