[Qemu-block] [PATCH 3/5] block/dirty-bitmap: change semantics of enabled predicate

[John Snow]

Currently, enabled means something like "the status of the bitmap
is ACTIVE." After this patch, it should mean exclusively: "This
bitmap is recording guest writes, and is allowed to do so."

In many places, this is how this predicate was already used.
We'll allow users to call user_locked if they're really curious about
finding out if the bitmap is in use by an operation.

To accommodate this, modify the create_successor routine to now
explicitly disable the parent bitmap at creation time.


Justifications:

1. bdrv_dirty_bitmap_status suffers no change from the lack of
   1:1 parity with the new predicates because of the order in which
   the predicates are checked. This is now only for compatibility.

2. bdrv_set_dirty_bitmap is only used by mirror, which does not use
   disabled bitmaps -- all of these writes are internal usages.
   Therefore, we should allow writes even in the disabled state.
   The condition is removed.

3. bdrv_reset_dirty_bitmap Similarly, this is only used internally by
   mirror and migration. In these contexts it is always enabled anyway,
   but our API does not need to enforce this.

4. bdrv_set_dirty will skip recording writes from the guest here if
   we are disabled OR if we had a successor, which now changes.
   Accommodate the change by explicitly disabling bitmaps with successors.

5. qcow2/dirty-bitmap: This only ever wanted to check if the bitmap
   was enabled or not. Theoretically if we save during an operation,
   this now gets set as enabled instead of disabled.

6. block_dirty_bitmap_enable_prepare only ever cared if about the
   literal bit, and already checked for user_locked beforehand.

7. block_dirty_bitmap_disable_prepare ditto as above.

8. init_dirty_bitmap_migration also already checks user_locked,
   so this call can be a simple enabled/disabled check.
---
 block/dirty-bitmap.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/block/dirty-bitmap.c b/block/dirty-bitmap.c
index dbe2d97d3f..1a433130ff 100644
--- a/block/dirty-bitmap.c
+++ b/block/dirty-bitmap.c
@@ -209,7 +209,7 @@ bool bdrv_dirty_bitmap_qmp_locked(BdrvDirtyBitmap *bitmap)
 /* Called with BQL taken.  */
 bool bdrv_dirty_bitmap_enabled(BdrvDirtyBitmap *bitmap)
 {
-    return !(bitmap->disabled || bitmap->successor);
+    return !bitmap->disabled;
 }
 
 /* Called with BQL taken.  */
@@ -264,6 +264,7 @@ int bdrv_dirty_bitmap_create_successor(BlockDriverState *bs,
 
     /* Successor will be on or off based on our current state. */
     child->disabled = bitmap->disabled;
+    bitmap->disabled = true;
 
     /* Install the successor and freeze the parent */
     bitmap->successor = child;
@@ -346,6 +347,8 @@ BdrvDirtyBitmap 
*bdrv_reclaim_dirty_bitmap_locked(BlockDriverState *bs,
         error_setg(errp, "Merging of parent and successor bitmap failed");
         return NULL;
     }
+
+    parent->disabled = successor->disabled;
     bdrv_release_dirty_bitmap_locked(successor);
     parent->successor = NULL;
 
@@ -542,7 +545,6 @@ int64_t bdrv_dirty_iter_next(BdrvDirtyBitmapIter *iter)
 void bdrv_set_dirty_bitmap_locked(BdrvDirtyBitmap *bitmap,
                                   int64_t offset, int64_t bytes)
 {
-    assert(bdrv_dirty_bitmap_enabled(bitmap));
     assert(!bdrv_dirty_bitmap_readonly(bitmap));
     hbitmap_set(bitmap->bitmap, offset, bytes);
 }
@@ -559,7 +561,6 @@ void bdrv_set_dirty_bitmap(BdrvDirtyBitmap *bitmap,
 void bdrv_reset_dirty_bitmap_locked(BdrvDirtyBitmap *bitmap,
                                     int64_t offset, int64_t bytes)
 {
-    assert(bdrv_dirty_bitmap_enabled(bitmap));
     assert(!bdrv_dirty_bitmap_readonly(bitmap));
     hbitmap_reset(bitmap->bitmap, offset, bytes);
 }
-- 
2.17.2