Re: [Qemu-devel] [PATCH v17 02/14] block: Introduce op_blockers to BlockDriverState

[Jeff Cody <jcody@redhat.com>]

On Mon, Mar 10, 2014 at 03:25:58PM +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().

Is there a reason to assume there will be atypical usages that don't
follow these steps?  If not, could the Error reason resource be
allocated inside the block() operation if non-NULL, and freed inside
the unblock() operations?

> 
> Signed-off-by: Fam Zheng <famz@redhat.com>
> Reviewed-by: Benoit Canet <benoit@irqsave.net>
> ---
>  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 f1ef4b0..56a4433 100644
> --- a/block.c
> +++ b/block.c
> @@ -335,6 +335,7 @@ void bdrv_register(BlockDriver *bdrv)
>  BlockDriverState *bdrv_new(const char *device_name)
>  {
>      BlockDriverState *bs;
> +    int i;
>  
>      bs = g_malloc0(sizeof(BlockDriverState));
>      QLIST_INIT(&bs->dirty_bitmaps);
> @@ -342,6 +343,9 @@ BlockDriverState *bdrv_new(const char *device_name)
>      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);
> @@ -1853,6 +1857,8 @@ 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));
>  }
>  
>  /*
> @@ -5151,6 +5157,75 @@ 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);
> +        }

The blocker assignment could be moved inside the if bracket (just an
observation, not a request)


Reviewed-by: Jeff Cody <jcody@redhat.com>


> +        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 8820735..4874e2a 100644
> --- a/include/block/block.h
> +++ b/include/block/block.h
> @@ -474,6 +474,13 @@ 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;
>  
> -- 
> 1.9.0
> 
>