block/backup: improve comment about image fleecing

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190920142056.12778-5-vsementsov@virtuozzo.com
Signed-off-by: Max Reitz <mreitz@redhat.com>

block/backup.c

@@ -747,9 +747,18 @@ BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
     job->bitmap_mode = bitmap_mode;
 
     /*
-     * Set write flags:
-     * 1. Detect image-fleecing (and similar) schemes
-     * 2. Handle compression
+     * If source is in backing chain of target assume that target is going to be
+     * used for "image fleecing", i.e. it should represent a kind of snapshot of
+     * source at backup-start point in time. And target is going to be read by
+     * somebody (for example, used as NBD export) during backup job.
+     *
+     * In this case, we need to add BDRV_REQ_SERIALISING write flag to avoid
+     * intersection of backup writes and third party reads from target,
+     * otherwise reading from target we may occasionally read already updated by
+     * guest data.
+     *
+     * For more information see commit f8d59dfb40bb and test
+     * tests/qemu-iotests/222
      */
     job->write_flags =
         (bdrv_chain_contains(target, bs) ? BDRV_REQ_SERIALISING : 0) |