block/dirty-bitmap: add bdrv_dirty_bitmap_merge_internal

I'm surprised it didn't come up sooner, but sometimes we have a +busy
bitmap as a source. This is dangerous from the QMP API, but if we are
the owner that marked the bitmap busy, it's safe to merge it using it as
a read only source.

It is not safe in the general case to allow users to read from in-use
bitmaps, so create an internal variant that foregoes the safety
checking.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190709232550.10724-10-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>

2 files changed, 52 insertions, 5 deletions

diff --git a/block/dirty-bitmap.c b/block/dirty-bitmap.c
index 95a9c2a..7881fea 100644
--- a/block/dirty-bitmap.c
+++ b/block/dirty-bitmap.c
@@ -810,6 +810,12 @@ bool bdrv_dirty_bitmap_next_dirty_area(BdrvDirtyBitmap *bitmap,
     return hbitmap_next_dirty_area(bitmap->bitmap, offset, bytes);
 }
 
+/**
+ * bdrv_merge_dirty_bitmap: merge src into dest.
+ * Ensures permissions on bitmaps are reasonable; use for public API.
+ *
+ * @backup: If provided, make a copy of dest here prior to merge.
+ */
 void bdrv_merge_dirty_bitmap(BdrvDirtyBitmap *dest, const BdrvDirtyBitmap *src,
                              HBitmap **backup, Error **errp)
 {
@@ -833,6 +839,42 @@ void bdrv_merge_dirty_bitmap(BdrvDirtyBitmap *dest, const BdrvDirtyBitmap *src,
         goto out;
     }
 
+    ret = bdrv_dirty_bitmap_merge_internal(dest, src, backup, false);
+    assert(ret);
+
+out:
+    qemu_mutex_unlock(dest->mutex);
+    if (src->mutex != dest->mutex) {
+        qemu_mutex_unlock(src->mutex);
+    }
+}
+
+/**
+ * bdrv_dirty_bitmap_merge_internal: merge src into dest.
+ * Does NOT check bitmap permissions; not suitable for use as public API.
+ *
+ * @backup: If provided, make a copy of dest here prior to merge.
+ * @lock: If true, lock and unlock bitmaps on the way in/out.
+ * returns true if the merge succeeded; false if unattempted.
+ */
+bool bdrv_dirty_bitmap_merge_internal(BdrvDirtyBitmap *dest,
+                                      const BdrvDirtyBitmap *src,
+                                      HBitmap **backup,
+                                      bool lock)
+{
+    bool ret;
+
+    assert(!bdrv_dirty_bitmap_readonly(dest));
+    assert(!bdrv_dirty_bitmap_inconsistent(dest));
+    assert(!bdrv_dirty_bitmap_inconsistent(src));
+
+    if (lock) {
+        qemu_mutex_lock(dest->mutex);
+        if (src->mutex != dest->mutex) {
+            qemu_mutex_lock(src->mutex);
+        }
+    }
+
     if (backup) {
         *backup = dest->bitmap;
         dest->bitmap = hbitmap_alloc(dest->size, hbitmap_granularity(*backup));
@@ -840,11 +882,13 @@ void bdrv_merge_dirty_bitmap(BdrvDirtyBitmap *dest, const BdrvDirtyBitmap *src,
     } else {
         ret = hbitmap_merge(dest->bitmap, src->bitmap, dest->bitmap);
     }
-    assert(ret);
 
-out:
-    qemu_mutex_unlock(dest->mutex);
-    if (src->mutex != dest->mutex) {
-        qemu_mutex_unlock(src->mutex);
+    if (lock) {
+        qemu_mutex_unlock(dest->mutex);
+        if (src->mutex != dest->mutex) {
+            qemu_mutex_unlock(src->mutex);
+        }
     }
+
+    return ret;
 }
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 80953ac8..aa697f1 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -1253,6 +1253,9 @@ void bdrv_set_dirty(BlockDriverState *bs, int64_t offset, int64_t bytes);
 
 void bdrv_clear_dirty_bitmap(BdrvDirtyBitmap *bitmap, HBitmap **out);
 void bdrv_restore_dirty_bitmap(BdrvDirtyBitmap *bitmap, HBitmap *backup);
+bool bdrv_dirty_bitmap_merge_internal(BdrvDirtyBitmap *dest,
+                                      const BdrvDirtyBitmap *src,
+                                      HBitmap **backup, bool lock);
 
 void bdrv_inc_in_flight(BlockDriverState *bs);
 void bdrv_dec_in_flight(BlockDriverState *bs);