Clarify first two cases and fix Modify -> Any access in third case.
Also, drop 'only' from third case, as it a bit confuses, when thinking
about case where we modify BdrvDirtyBitmap and access HBitmap.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 include/block/block_int.h | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/include/block/block_int.h b/include/block/block_int.h
index 27e17addba..e5af7820ff 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -701,10 +701,14 @@ struct BlockDriverState {
     uint64_t write_threshold_offset;
     NotifierWithReturn write_threshold_notifier;
 
-    /* Writing to the list requires the BQL _and_ the dirty_bitmap_mutex.
-     * Reading from the list can be done with either the BQL or the
-     * dirty_bitmap_mutex.  Modifying a bitmap only requires
-     * dirty_bitmap_mutex.  */
+    /* Writing to the list (i.e. to any field of BdrvDirtyBitmap or to the
+     * list-head) requires both the BQL _and_ the dirty_bitmap_mutex.
+     *
+     * Reading from the list (from any field of BdrvDirtyBitmap or from the
+     * list-head) can be done with either the BQL or the dirty_bitmap_mutex.
+     *
+     * Any access to underlying HBitmap requires dirty_bitmap_mutex.
+     */
     QemuMutex dirty_bitmap_mutex;
     QLIST_HEAD(, BdrvDirtyBitmap) dirty_bitmaps;
 
-- 
2.11.1