[Qemu-devel] [PATCH v7 04/10] block: support dropping active in bdrv_drop_intermediate

[Fam Zheng]

Dropping intermediate could be useful both for commit and stream, and
BDS refcnt plus bdrv_swap could do most of the job nicely. It also need
some improvements in preparation for op blockers.

Signed-off-by: Fam Zheng <address@hidden>
---
 block.c        | 152 +++++++++++++++++++++++++++------------------------------
 block/commit.c |   1 +
 2 files changed, 74 insertions(+), 79 deletions(-)

diff --git a/block.c b/block.c
index 41562fd..681d3be 100644
--- a/block.c
+++ b/block.c
@@ -2163,114 +2163,108 @@ BlockDriverState *bdrv_find_overlay(BlockDriverState 
*active,
     return overlay;
 }
 
-typedef struct BlkIntermediateStates {
-    BlockDriverState *bs;
-    QSIMPLEQ_ENTRY(BlkIntermediateStates) entry;
-} BlkIntermediateStates;
-
+static void bdrv_set_backing_hd(BlockDriverState *bs,
+                                BlockDriverState *new_backing)
+{
+    if (bs->backing_hd) {
+        bdrv_op_unblock_all(bs->backing_hd, bs->backing_blocker);
+    }
+    bs->backing_hd = new_backing;
+    if (new_backing) {
+        bdrv_op_block_all(bs->backing_hd, bs->backing_blocker);
+    }
+}
 
 /*
- * Drops images above 'base' up to and including 'top', and sets the image
- * above 'top' to have base as its backing file.
+ * Drops images above 'base' up to and including 'top', and sets new 'base'
+ * as backing_hd of top_overlay (the image orignally has 'top' as backing
+ * file). top_overlay may be NULL if 'top' is active, no such update needed.
+ * Requires that the top_overlay to 'top' is opened r/w.
  *
- * Requires that the overlay to 'top' is opened r/w, so that the backing file
- * information in 'bs' can be properly updated.
+ * 1) This will convert the following chain:
+ * ... <- base <- ... <- top <- overlay <-... <- active
  *
- * E.g., this will convert the following chain:
- * bottom <- base <- intermediate <- top <- active
+ * to
+ *
+ * ... <- base <- overlay <- active
+ *
+ * 2) It is allowed for bottom==base, in which case it converts:
+ *
+ * ... <- base <- ... <- top <- overlay <- ... <- active
  *
  * to
  *
- * bottom <- base <- active
+ * base <- overlay <- active
  *
- * It is allowed for bottom==base, in which case it converts:
+ * 2) It also allows active==top, in which case it converts:
  *
- * base <- intermediate <- top <- active
+ * ... <- base <- ... <- top (active)
  *
  * to
  *
- * base <- active
+ * base == active == top, i.e. only base and lower remains: *top == *base when
+ * return.
+ *
+ * 3) If base==NULL, it will drop all the BDS below overlay and set its
+ * backing_hd to NULL. I.e.:
  *
- * Error conditions:
- *  if active == top, that is considered an error
+ * base(NULL) <- ... <- overlay <- ... <- active
  *
+ * to
+ *
+ * overlay <- ... <- active
  */
 int bdrv_drop_intermediate(BlockDriverState *active, BlockDriverState *top,
                            BlockDriverState *base)
 {
-    BlockDriverState *intermediate;
-    BlockDriverState *base_bs = NULL;
-    BlockDriverState *new_top_bs = NULL;
-    BlkIntermediateStates *intermediate_state, *next;
-    int ret = -EIO;
-
-    QSIMPLEQ_HEAD(states_to_delete, BlkIntermediateStates) states_to_delete;
-    QSIMPLEQ_INIT(&states_to_delete);
-
-    if (!top->drv || !base->drv) {
-        goto exit;
-    }
+    BlockDriverState *drop_start, *overlay;
+    int ret = -EINVAL;
 
-    new_top_bs = bdrv_find_overlay(active, top);
-
-    if (new_top_bs == NULL) {
-        /* we could not find the image above 'top', this is an error */
+    if (!top->drv || (base && !base->drv)) {
         goto exit;
     }
-
-    /* special case of new_top_bs->backing_hd already pointing to base - 
nothing
-     * to do, no intermediate images */
-    if (new_top_bs->backing_hd == base) {
+    if (top == base) {
         ret = 0;
-        goto exit;
-    }
-
-    intermediate = top;
-
-    /* now we will go down through the list, and add each BDS we find
-     * into our deletion queue, until we hit the 'base'
-     */
-    while (intermediate) {
-        intermediate_state = g_malloc0(sizeof(BlkIntermediateStates));
-        intermediate_state->bs = intermediate;
-        QSIMPLEQ_INSERT_TAIL(&states_to_delete, intermediate_state, entry);
-
-        if (intermediate->backing_hd == base) {
-            base_bs = intermediate->backing_hd;
-            break;
+    } else if (top == active) {
+        assert(base);
+        drop_start = active->backing_hd;
+        bdrv_swap(active, base);
+        base->backing_hd = NULL;
+        bdrv_unref(drop_start);
+        ret = 0;
+    } else {
+        /* If there's an overlay, its backing_hd points to top's BDS now,
+         * the top image is dropped but this BDS structure is kept and swapped
+         * with base, this way we keep the pointers valid after dropping top */
+        overlay = bdrv_find_overlay(active, top);
+        if (!overlay) {
+            goto exit;
+        }
+        if (base) {
+            ret = bdrv_change_backing_file(overlay, base->filename,
+                                           base->drv->format_name);
+        } else {
+            ret = bdrv_change_backing_file(overlay, "", "");
+        }
+        if (ret) {
+            goto exit;
+        }
+        if (base) {
+            drop_start = top->backing_hd;
+            bdrv_swap(top, base);
+            /* Break the loop formed by bdrv_swap */
+            bdrv_set_backing_hd(base, NULL);
+        } else {
+            bdrv_set_backing_hd(overlay, NULL);
+            drop_start = top;
         }
-        intermediate = intermediate->backing_hd;
-    }
-    if (base_bs == NULL) {
-        /* something went wrong, we did not end at the base. safely
-         * unravel everything, and exit with error */
-        goto exit;
-    }
-
-    /* success - we can delete the intermediate states, and link top->base */
-    ret = bdrv_change_backing_file(new_top_bs, base_bs->filename,
-                                   base_bs->drv ? base_bs->drv->format_name : 
"");
-    if (ret) {
-        goto exit;
-    }
-    new_top_bs->backing_hd = base_bs;
-
 
-    QSIMPLEQ_FOREACH_SAFE(intermediate_state, &states_to_delete, entry, next) {
-        /* so that bdrv_close() does not recursively close the chain */
-        intermediate_state->bs->backing_hd = NULL;
-        bdrv_unref(intermediate_state->bs);
+        bdrv_unref(drop_start);
     }
-    ret = 0;
-
 exit:
-    QSIMPLEQ_FOREACH_SAFE(intermediate_state, &states_to_delete, entry, next) {
-        g_free(intermediate_state);
-    }
     return ret;
 }
 
-
 static int bdrv_check_byte_request(BlockDriverState *bs, int64_t offset,
                                    size_t size)
 {
diff --git a/block/commit.c b/block/commit.c
index d4090cb..4d8cd05 100644
--- a/block/commit.c
+++ b/block/commit.c
@@ -142,6 +142,7 @@ wait:
     if (!block_job_is_cancelled(&s->common) && sector_num == end) {
         /* success */
         ret = bdrv_drop_intermediate(active, top, base);
+        base = top;
     }
 
 exit_free_buf:
-- 
1.8.5.1