Re: [Qemu-devel] [PATCH 2/3] block: adjust qmp_transaction to be extendable

[Wenchao Xia]

于 2013-4-2 21:55, Kevin Wolf 写道:
> Am 01.04.2013 um 12:01 hat Wenchao Xia geschrieben:
> >    Now code for external snapshot are packaged as one case
> > in qmp_transaction, so later other operation could be added.
> >    The logic in qmp_transaction is changed a bit: Original code
> > tries to create all images first and then update all *bdrv
> > once together, new code create and update one *bdrv one time,
> > and use bdrv_deappend() to rollback on fail. This allows mixing
> > different kind of requests in qmp_transaction() later.
> >
> > Signed-off-by: Wenchao Xia <address@hidden>
> > ---
> >   blockdev.c |  250 +++++++++++++++++++++++++++++++++++++-----------------------
> >   1 files changed, 153 insertions(+), 97 deletions(-)
> >
> > diff --git a/blockdev.c b/blockdev.c
> > index 8cdc9ce..75416fb 100644
> > --- a/blockdev.c
> > +++ b/blockdev.c
> > @@ -779,9 +779,155 @@ void qmp_blockdev_snapshot_sync(const char *device, const 
> > char *snapshot_file,
> >
> >
> >   /* New and old BlockDriverState structs for group snapshots */
> > -typedef struct BlkTransactionStates {
> > +typedef struct BdrvActionOps {
> > +    int (*commit)(BlockdevAction *action, void **p_opaque, Error **errp);
> > +    void (*rollback)(BlockdevAction *action, void *opaque);
> > +    void (*clean)(BlockdevAction *action, void *opaque);
> > +} BdrvActionOps;
>
> You don't really implement the transactional pattern that was used by
> the original code (and is used elsewhere). It should work like this:
>
> 1. Prepare: This stage performs all operations that can fail. They are
>     not made active yet. For example with snapshotting, open a new
>     BlockDriver state, but don't change the backing file chain yet.
>
> 2. Commit: Activate the change. This operation can never fail. For this
>     reason, you never have to undo anything done here.
>
> 3. Rollback: Basically just free everything that prepare did up to the
>     error.
>
> If you do it your way, you get into serious trouble if rollback involves
> operations that can fail.
>
> In the original code, here start the prepare:
  That is a clear comment, thanks. I changed the model since expecting
commit operation may need rollback, if not I am confident to keep
original model.
  Since bdrv_snapshot_delete() can fail, I guess assertion of its
success should be used in the rollback later.

> > @@ -806,125 +950,37 @@ void qmp_transaction(BlockdevActionList *dev_list, Error 
> > **errp)
> >       /* We don't do anything in this loop that commits us to the snapshot */
> >       while (NULL != dev_entry) {
> >           BlockdevAction *dev_info = NULL;
> > -        BlockDriver *proto_drv;
> > -        BlockDriver *drv;
> > -        int flags;
> > -        enum NewImageMode mode;
> > -        const char *new_image_file;
> > -        const char *device;
> > -        const char *format = "qcow2";
> > -
> >           dev_info = dev_entry->value;
> >           dev_entry = dev_entry->next;
> >
> >           states = g_malloc0(sizeof(BlkTransactionStates));
> >           QSIMPLEQ_INSERT_TAIL(&snap_bdrv_states, states, entry);
> >
> > +        states->action = dev_info;
> >           switch (dev_info->kind) {
> >           case BLOCKDEV_ACTION_KIND_BLOCKDEV_SNAPSHOT_SYNC:
> > -            device = dev_info->blockdev_snapshot_sync->device;
> > -            if (!dev_info->blockdev_snapshot_sync->has_mode) {
> > -                dev_info->blockdev_snapshot_sync->mode = 
> > NEW_IMAGE_MODE_ABSOLUTE_PATHS;
> > -            }
> > -            new_image_file = dev_info->blockdev_snapshot_sync->snapshot_file;
> > -            if (dev_info->blockdev_snapshot_sync->has_format) {
> > -                format = dev_info->blockdev_snapshot_sync->format;
> > -            }
> > -            mode = dev_info->blockdev_snapshot_sync->mode;
> > +            states->ops = &external_snapshot_ops;
> >               break;
> >           default:
> >               abort();
> >           }
> >
> > -        drv = bdrv_find_format(format);
> > -        if (!drv) {
> > -            error_set(errp, QERR_INVALID_BLOCK_FORMAT, format);
> > -            goto delete_and_fail;
> > -        }
> > -
> > -        states->old_bs = bdrv_find(device);
> > -        if (!states->old_bs) {
> > -            error_set(errp, QERR_DEVICE_NOT_FOUND, device);
> > -            goto delete_and_fail;
> > -        }
> > -
> > -        if (!bdrv_is_inserted(states->old_bs)) {
> > -            error_set(errp, QERR_DEVICE_HAS_NO_MEDIUM, device);
> > -            goto delete_and_fail;
> > -        }
> > -
> > -        if (bdrv_in_use(states->old_bs)) {
> > -            error_set(errp, QERR_DEVICE_IN_USE, device);
> > -            goto delete_and_fail;
> > -        }
> > -
> > -        if (!bdrv_is_read_only(states->old_bs)) {
> > -            if (bdrv_flush(states->old_bs)) {
> > -                error_set(errp, QERR_IO_ERROR);
> > -                goto delete_and_fail;
> > -            }
> > -        }
> > -
> > -        flags = states->old_bs->open_flags;
> > -
> > -        proto_drv = bdrv_find_protocol(new_image_file);
> > -        if (!proto_drv) {
> > -            error_set(errp, QERR_INVALID_BLOCK_FORMAT, format);
> > -            goto delete_and_fail;
> > -        }
> > -
> > -        /* create new image w/backing file */
> > -        if (mode != NEW_IMAGE_MODE_EXISTING) {
> > -            bdrv_img_create(new_image_file, format,
> > -                            states->old_bs->filename,
> > -                            states->old_bs->drv->format_name,
> > -                            NULL, -1, flags, &local_err, false);
> > -            if (error_is_set(&local_err)) {
> > -                error_propagate(errp, local_err);
> > -                goto delete_and_fail;
> > -            }
> > -        }
> > -
> > -        /* We will manually add the backing_hd field to the bs later */
> > -        states->new_bs = bdrv_new("");
> > -        /* TODO Inherit bs->options or only take explicit options with an
> > -         * extended QMP command? */
> > -        ret = bdrv_open(states->new_bs, new_image_file, NULL,
> > -                        flags | BDRV_O_NO_BACKING, drv);
> > -        if (ret != 0) {
> > -            error_set(errp, QERR_OPEN_FILE_FAILED, new_image_file);
> > +        if (states->ops->commit(states->action, &states->opaque, errp)) {
> >               goto delete_and_fail;
> >           }
> >       }
>
> The following block is the commit:
>
> > -
> > -    /* Now we are going to do the actual pivot.  Everything up to this point
> > -     * is reversible, but we are committed at this point */
> > -    QSIMPLEQ_FOREACH(states, &snap_bdrv_states, entry) {
> > -        /* This removes our old bs from the bdrv_states, and adds the new bs */
> > -        bdrv_append(states->new_bs, states->old_bs);
> > -        /* We don't need (or want) to use the transactional
> > -         * bdrv_reopen_multiple() across all the entries at once, because we
> > -         * don't want to abort all of them if one of them fails the reopen */
> > -        bdrv_reopen(states->new_bs, states->new_bs->open_flags & ~BDRV_O_RDWR,
> > -                    NULL);
> > -    }
> > -
> >       /* success */
> >       goto exit;
>
> And this is rollback:
>
> >   delete_and_fail:
> > -    /*
> > -    * failure, and it is all-or-none; abandon each new bs, and keep using
> > -    * the original bs for all images
> > -    */
> >       QSIMPLEQ_FOREACH(states, &snap_bdrv_states, entry) {
> > -        if (states->new_bs) {
> > -             bdrv_delete(states->new_bs);
> > -        }
> > +        states->ops->rollback(states->action, states->opaque);
> >       }
>
> Kevin


--
Best Regards

Wenchao Xia