Re: [Qemu-devel] [PATCH v13 02/14] block: Introduce op_blockers to BlockDriverState

["Benoît Canet" <benoit.canet@irqsave.net>]

The Wednesday 29 Jan 2014 à 13:07:29 (+0800), Fam Zheng wrote :
> 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>
> ---
>  block.c                   | 71 +++++++++++++++++++++++++++++++++++++++++++++++
>  include/block/block.h     |  7 +++++
>  include/block/block_int.h |  5 ++++
>  3 files changed, 83 insertions(+)
> 
> diff --git a/block.c b/block.c
> index cb21a5f..967de98 100644
> --- a/block.c
> +++ b/block.c
> @@ -1838,6 +1838,8 @@ static void bdrv_move_feature_fields(BlockDriverState *bs_dest,
>       * We do want to swap name but don't want to swap linked list entries
>       */
>      bs_dest->node_list   = bs_src->node_list;
> +    memcpy(bs_dest->op_blockers, bs_src->op_blockers,
> +           sizeof(bs_dest->op_blockers));
>  }
>  
>  /*
> @@ -5119,6 +5121,75 @@ 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);
> +    if (!QLIST_EMPTY(&bs->op_blockers[op])) {
> +        blocker = QLIST_FIRST(&bs->op_blockers[op]);
> +        if (errp) {
> +            *errp = error_copy(blocker->reason);

When an operation is blocked the first reason found is returned as **errp.

I think that this could lead to some randomization of the error messages
depending on the bdrv_op_block call order.

> +        }
> +        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));
> +    blocker->reason = reason;
> +    QLIST_INSERT_HEAD(&bs->op_blockers[op], blocker, list);
> +}
> +
> +void bdrv_op_unblock(BlockDriverState *bs, BlockOpType op, Error *reason)
> +{
> +    BdrvOpBlocker *blocker, *next;
> +    assert((int) op >= 0 && op < BLOCK_OP_TYPE_MAX);
> +    QLIST_FOREACH_SAFE(blocker, &bs->op_blockers[op], list, next) {
> +        if (blocker->reason == reason) {
> +            QLIST_REMOVE(blocker, list);
> +            g_free(blocker);
> +        }
> +    }
> +}
> +
> +void bdrv_op_block_all(BlockDriverState *bs, Error *reason)
> +{
> +    int i;
> +    for (i = 0; i < BLOCK_OP_TYPE_MAX; i++) {
> +        bdrv_op_block(bs, i, reason);
> +    }
> +}
> +
> +void bdrv_op_unblock_all(BlockDriverState *bs, Error *reason)
> +{
> +    int i;
> +    for (i = 0; i < BLOCK_OP_TYPE_MAX; i++) {
> +        bdrv_op_unblock(bs, i, reason);
> +    }
> +}
> +
> +bool bdrv_op_blocker_is_empty(BlockDriverState *bs)
> +{
> +    int i;
> +
> +    for (i = 0; i < BLOCK_OP_TYPE_MAX; i++) {
> +        if (!QLIST_EMPTY(&bs->op_blockers[i])) {
> +            return false;
> +        }
> +    }
> +    return true;
> +}
> +
>  void bdrv_set_in_use(BlockDriverState *bs, int in_use)
>  {
>      assert(bs->in_use != in_use);
> diff --git a/include/block/block.h b/include/block/block.h
> index 9626650..f4a70c9 100644
> --- a/include/block/block.h
> +++ b/include/block/block.h
> @@ -473,6 +473,13 @@ void bdrv_unref(BlockDriverState *bs);
>  void bdrv_set_in_use(BlockDriverState *bs, int in_use);
>  int bdrv_in_use(BlockDriverState *bs);
>  
> +bool bdrv_op_is_blocked(BlockDriverState *bs, BlockOpType op, Error **errp);
> +void bdrv_op_block(BlockDriverState *bs, BlockOpType op, Error *reason);
> +void bdrv_op_unblock(BlockDriverState *bs, BlockOpType op, Error *reason);
> +void bdrv_op_block_all(BlockDriverState *bs, Error *reason);
> +void bdrv_op_unblock_all(BlockDriverState *bs, Error *reason);
> +bool bdrv_op_blocker_is_empty(BlockDriverState *bs);
> +
>  #ifdef CONFIG_LINUX_AIO
>  int raw_get_aio_fd(BlockDriverState *bs);
>  #else
> diff --git a/include/block/block_int.h b/include/block/block_int.h
> index 0bcf1c9..4e558d0 100644
> --- a/include/block/block_int.h
> +++ b/include/block/block_int.h
> @@ -270,6 +270,8 @@ typedef struct BlockLimits {
>      size_t opt_mem_alignment;
>  } BlockLimits;
>  
> +typedef struct BdrvOpBlocker BdrvOpBlocker;
> +
>  /*
>   * Note: the function bdrv_append() copies and swaps contents of
>   * BlockDriverStates, so if you add new fields to this struct, please
> @@ -361,6 +363,9 @@ struct BlockDriverState {
>  
>      QLIST_HEAD(, BdrvTrackedRequest) tracked_requests;
>  
> +    /* operation blockers */
> +    QLIST_HEAD(, BdrvOpBlocker) op_blockers[BLOCK_OP_TYPE_MAX];
> +
>      /* long-running background operation */
>      BlockJob *job;
>  
> -- 
> 1.8.5.3
> 
>