Re: [Qemu-devel] [PATCH v19 03/16] block: Introduce op_blockers to BlockDriverState

[Markus Armbruster <armbru@redhat.com>]

Fam Zheng <famz@redhat.com> writes:

> BlockDriverState.op_blockers is an array of lists with BLOCK_OP_TYPE_MAX
> elements. Each list is a list of blockers of an operation type
> (BlockOpType), that marks this BDS as currently blocked for a certain
> type of operation with reason errors stored in the list. The rule of
> usage is:
>
>  * BDS user who wants to take an operation should check if there's any
>    blocker of the type with bdrv_op_is_blocked().
>
>  * BDS user who wants to block certain types of operation, should call
>    bdrv_op_block (or bdrv_op_block_all to block all types of operations,
>    which is similar to the existing bdrv_set_in_use()).
>
>  * A blocker is only referenced by op_blockers, so the lifecycle is
>    managed by caller, and shouldn't be lost until unblock, so typically
>    a caller does these:
>
>    - Allocate a blocker with error_setg or similar, call bdrv_op_block()
>      to block some operations.
>    - Hold the blocker, do his job.
>    - Unblock operations that it blocked, with the same reason pointer
>      passed to bdrv_op_unblock().
>    - Release the blocker with error_free().
>
> Signed-off-by: Fam Zheng <famz@redhat.com>
> Reviewed-by: Benoit Canet <benoit@irqsave.net>
> Reviewed-by: Jeff Cody <jcody@redhat.com>
> ---
>  block.c                   | 75 +++++++++++++++++++++++++++++++++++++++++++++++
>  include/block/block.h     |  7 +++++
>  include/block/block_int.h |  5 ++++
>  3 files changed, 87 insertions(+)
>
> diff --git a/block.c b/block.c
> index b749d31..32338ca 100644
> --- a/block.c
> +++ b/block.c
> @@ -335,6 +335,7 @@ void bdrv_register(BlockDriver *bdrv)
>  BlockDriverState *bdrv_new(const char *device_name, Error **errp)
>  {
>      BlockDriverState *bs;
> +    int i;
>  
>      if (bdrv_find(device_name)) {
>          error_setg(errp, "Device with id '%s' already exists",
> @@ -353,6 +354,9 @@ BlockDriverState *bdrv_new(const char *device_name, Error **errp)
>      if (device_name[0] != '\0') {
>          QTAILQ_INSERT_TAIL(&bdrv_states, bs, device_list);
>      }
> +    for (i = 0; i < BLOCK_OP_TYPE_MAX; i++) {
> +        QLIST_INIT(&bs->op_blockers[i]);
> +    }
>      bdrv_iostatus_disable(bs);
>      notifier_list_init(&bs->close_notifiers);
>      notifier_with_return_list_init(&bs->before_write_notifiers);
> @@ -1907,6 +1911,8 @@ static void bdrv_move_feature_fields(BlockDriverState *bs_dest,
>      pstrcpy(bs_dest->device_name, sizeof(bs_dest->device_name),
>              bs_src->device_name);
>      bs_dest->device_list = bs_src->device_list;
> +    memcpy(bs_dest->op_blockers, bs_src->op_blockers,
> +           sizeof(bs_dest->op_blockers));
>  }
>  
>  /*
> @@ -5269,6 +5275,75 @@ void bdrv_unref(BlockDriverState *bs)
>      }
>  }
>  
> +struct BdrvOpBlocker {
> +    Error *reason;
> +    QLIST_ENTRY(BdrvOpBlocker) list;
> +};
> +
> +bool bdrv_op_is_blocked(BlockDriverState *bs, BlockOpType op, Error **errp)
> +{
> +    BdrvOpBlocker *blocker;
> +    assert((int) op >= 0 && op < BLOCK_OP_TYPE_MAX);

Space between cast and its operand is unusual.  Please don't respin just
for that.

> +    if (!QLIST_EMPTY(&bs->op_blockers[op])) {
> +        blocker = QLIST_FIRST(&bs->op_blockers[op]);
> +        if (errp) {
> +            *errp = error_copy(blocker->reason);
> +        }
> +        return true;
> +    }
> +    return false;
> +}
> +
> +void bdrv_op_block(BlockDriverState *bs, BlockOpType op, Error *reason)
> +{
> +    BdrvOpBlocker *blocker;
> +    assert((int) op >= 0 && op < BLOCK_OP_TYPE_MAX);
> +
> +    blocker = g_malloc0(sizeof(BdrvOpBlocker));

Please consider g_new0(), but don't respin just for that.

> +    blocker->reason = reason;
> +    QLIST_INSERT_HEAD(&bs->op_blockers[op], blocker, list);
> +}
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