Allow block backends to forward drain requests to their devices/users.
The initial intended purpose for this patch is to allow BBs to forward
requests along to BlockJobs, which will want to pause if their associated
BB has entered a drained region.

Signed-off-by: John Snow <jsnow@redhat.com>
---
 block/block-backend.c          | 24 ++++++++++++++++++++++--
 include/sysemu/block-backend.h |  8 ++++++++
 2 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/block/block-backend.c b/block/block-backend.c
index 5742c09..dec4a1c 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -65,6 +65,8 @@ struct BlockBackend {
     bool allow_write_beyond_eof;
 
     NotifierList remove_bs_notifiers, insert_bs_notifiers;
+
+    int quiesce_counter;
 };
 
 typedef struct BlockBackendAIOCB {
@@ -699,12 +701,17 @@ void blk_set_dev_ops(BlockBackend *blk, const BlockDevOps *ops,
                      void *opaque)
 {
     /* All drivers that use blk_set_dev_ops() are qdevified and we want to keep
-     * it that way, so we can assume blk->dev is a DeviceState if blk->dev_ops
-     * is set. */
+     * it that way, so we can assume blk->dev, if present, is a DeviceState if
+     * blk->dev_ops is set. Non-device users may use dev_ops without device. */
     assert(!blk->legacy_dev);
 
     blk->dev_ops = ops;
     blk->dev_opaque = opaque;
+
+    /* Are we currently quiesced? Should we enforce this right now? */
+    if (blk->quiesce_counter && ops->drained_begin) {
+        ops->drained_begin(opaque);
+    }
 }
 
 /*
@@ -1870,6 +1877,12 @@ static void blk_root_drained_begin(BdrvChild *child)
 {
     BlockBackend *blk = child->opaque;
 
+    if (++blk->quiesce_counter == 1) {
+        if (blk->dev_ops && blk->dev_ops->drained_begin) {
+            blk->dev_ops->drained_begin(blk->dev_opaque);
+        }
+    }
+
     /* Note that blk->root may not be accessible here yet if we are just
      * attaching to a BlockDriverState that is drained. Use child instead. */
 
@@ -1881,7 +1894,14 @@ static void blk_root_drained_begin(BdrvChild *child)
 static void blk_root_drained_end(BdrvChild *child)
 {
     BlockBackend *blk = child->opaque;
+    assert(blk->quiesce_counter);
 
     assert(blk->public.io_limits_disabled);
     --blk->public.io_limits_disabled;
+
+    if (--blk->quiesce_counter == 0) {
+        if (blk->dev_ops && blk->dev_ops->drained_end) {
+              blk->dev_ops->drained_end(blk->dev_opaque);
+        }
+    }
 }
diff --git a/include/sysemu/block-backend.h b/include/sysemu/block-backend.h
index 096c17f..7462228 100644
--- a/include/sysemu/block-backend.h
+++ b/include/sysemu/block-backend.h
@@ -58,6 +58,14 @@ typedef struct BlockDevOps {
      * Runs when the size changed (e.g. monitor command block_resize)
      */
     void (*resize_cb)(void *opaque);
+    /*
+     * Runs when the backend receives a drain request.
+     */
+    void (*drained_begin)(void *opaque);
+    /*
+     * Runs when the backend's last drain request ends.
+     */
+    void (*drained_end)(void *opaque);
 } BlockDevOps;
 
 /* This struct is embedded in (the private) BlockBackend struct and contains
-- 
2.9.3

On Thu, Mar 16, 2017 at 05:23:50PM -0400, John Snow wrote:
> Allow block backends to forward drain requests to their devices/users.
> The initial intended purpose for this patch is to allow BBs to forward
> requests along to BlockJobs, which will want to pause if their associated
> BB has entered a drained region.
> 
> Signed-off-by: John Snow <jsnow@redhat.com>
> ---
>  block/block-backend.c          | 24 ++++++++++++++++++++++--
>  include/sysemu/block-backend.h |  8 ++++++++
>  2 files changed, 30 insertions(+), 2 deletions(-)
> 
> diff --git a/block/block-backend.c b/block/block-backend.c
> index 5742c09..dec4a1c 100644
> --- a/block/block-backend.c
> +++ b/block/block-backend.c
> @@ -65,6 +65,8 @@ struct BlockBackend {
>      bool allow_write_beyond_eof;
>  
>      NotifierList remove_bs_notifiers, insert_bs_notifiers;
> +
> +    int quiesce_counter;
>  };
>  
>  typedef struct BlockBackendAIOCB {
> @@ -699,12 +701,17 @@ void blk_set_dev_ops(BlockBackend *blk, const BlockDevOps *ops,
>                       void *opaque)
>  {
>      /* All drivers that use blk_set_dev_ops() are qdevified and we want to keep
> -     * it that way, so we can assume blk->dev is a DeviceState if blk->dev_ops
> -     * is set. */
> +     * it that way, so we can assume blk->dev, if present, is a DeviceState if
> +     * blk->dev_ops is set. Non-device users may use dev_ops without device. */
>      assert(!blk->legacy_dev);
>  
>      blk->dev_ops = ops;
>      blk->dev_opaque = opaque;
> +
> +    /* Are we currently quiesced? Should we enforce this right now? */
> +    if (blk->quiesce_counter && ops->drained_begin) {
> +        ops->drained_begin(opaque);
> +    }
>  }
>  
>  /*
> @@ -1870,6 +1877,12 @@ static void blk_root_drained_begin(BdrvChild *child)
>  {
>      BlockBackend *blk = child->opaque;
>  
> +    if (++blk->quiesce_counter == 1) {
> +        if (blk->dev_ops && blk->dev_ops->drained_begin) {
> +            blk->dev_ops->drained_begin(blk->dev_opaque);
> +        }
> +    }
> +
>      /* Note that blk->root may not be accessible here yet if we are just
>       * attaching to a BlockDriverState that is drained. Use child instead. */
>  
> @@ -1881,7 +1894,14 @@ static void blk_root_drained_begin(BdrvChild *child)
>  static void blk_root_drained_end(BdrvChild *child)
>  {
>      BlockBackend *blk = child->opaque;
> +    assert(blk->quiesce_counter);
>  
>      assert(blk->public.io_limits_disabled);
>      --blk->public.io_limits_disabled;
> +
> +    if (--blk->quiesce_counter == 0) {
> +        if (blk->dev_ops && blk->dev_ops->drained_end) {
> +              blk->dev_ops->drained_end(blk->dev_opaque);
> +        }
> +    }
>  }
> diff --git a/include/sysemu/block-backend.h b/include/sysemu/block-backend.h
> index 096c17f..7462228 100644
> --- a/include/sysemu/block-backend.h
> +++ b/include/sysemu/block-backend.h
> @@ -58,6 +58,14 @@ typedef struct BlockDevOps {
>       * Runs when the size changed (e.g. monitor command block_resize)
>       */
>      void (*resize_cb)(void *opaque);
> +    /*
> +     * Runs when the backend receives a drain request.
> +     */
> +    void (*drained_begin)(void *opaque);
> +    /*
> +     * Runs when the backend's last drain request ends.
> +     */
> +    void (*drained_end)(void *opaque);
>  } BlockDevOps;
>  
>  /* This struct is embedded in (the private) BlockBackend struct and contains
> -- 
> 2.9.3
>

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