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Re: [PATCH 6/7] block: Let replace_child_noperm free children

From: Hanna Reitz
Subject: Re: [PATCH 6/7] block: Let replace_child_noperm free children
Date: Mon, 8 Nov 2021 11:12:52 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.2.0

On 05.11.21 16:41, Kevin Wolf wrote:
Am 04.11.2021 um 11:38 hat Hanna Reitz geschrieben:
In most of the block layer, especially when traversing down from other
BlockDriverStates, we assume that BdrvChild.bs can never be NULL.  When
it becomes NULL, it is expected that the corresponding BdrvChild pointer
also becomes NULL and the BdrvChild object is freed.

Therefore, once bdrv_replace_child_noperm() sets the BdrvChild.bs
pointer to NULL, it should also immediately set the corresponding
BdrvChild pointer (like bs->file or bs->backing) to NULL.

In that context, it also makes sense for this function to free the
child.  Sometimes we cannot do so, though, because it is called in a
transactional context where the caller might still want to reinstate the
child in the abort branch (and free it only on commit), so this behavior
has to remain optional.

In bdrv_replace_child_tran()'s abort handler, we now rely on the fact
that the BdrvChild passed to bdrv_replace_child_tran() must have had a
non-NULL .bs pointer initially.  Make a note of that and assert it.

Signed-off-by: Hanna Reitz <hreitz@redhat.com>
  block.c | 102 +++++++++++++++++++++++++++++++++++++++++++++++---------
  1 file changed, 86 insertions(+), 16 deletions(-)

diff --git a/block.c b/block.c
index ff45447686..0a85ede8dc 100644
--- a/block.c
+++ b/block.c
@@ -87,8 +87,10 @@ static BlockDriverState *bdrv_open_inherit(const char 
  static bool bdrv_recurse_has_child(BlockDriverState *bs,
                                     BlockDriverState *child);
+static void bdrv_child_free(BdrvChild *child);
  static void bdrv_replace_child_noperm(BdrvChild **child,
-                                      BlockDriverState *new_bs);
+                                      BlockDriverState *new_bs,
+                                      bool free_empty_child);
  static void bdrv_remove_file_or_backing_child(BlockDriverState *bs,
                                                BdrvChild *child,
                                                Transaction *tran);
@@ -2256,12 +2258,16 @@ typedef struct BdrvReplaceChildState {
      BdrvChild *child;
      BdrvChild **childp;
      BlockDriverState *old_bs;
+    bool free_empty_child;
  } BdrvReplaceChildState;
static void bdrv_replace_child_commit(void *opaque)
      BdrvReplaceChildState *s = opaque;
+ if (s->free_empty_child && !s->child->bs) {
+        bdrv_child_free(s->child);
+    }
@@ -2270,8 +2276,23 @@ static void bdrv_replace_child_abort(void *opaque)
      BdrvReplaceChildState *s = opaque;
      BlockDriverState *new_bs = s->child->bs;
- /* old_bs reference is transparently moved from @s to *s->childp */
-    bdrv_replace_child_noperm(s->childp, s->old_bs);
+    /*
+     * old_bs reference is transparently moved from @s to s->child;
+     * pass &s->child here instead of s->childp, because *s->childp
+     * will have been cleared by bdrv_replace_child_tran()'s
+     * bdrv_replace_child_noperm() call if new_bs is NULL, and we must
+     * not pass a NULL *s->childp here.
+     */
+    bdrv_replace_child_noperm(&s->child, s->old_bs, true);
Passing free_empty_child=true with a non-NULL new_bs looks a bit
confusing because the child isn't supposed to become empty anyway.

I wasn’t sure what to do.  I decided to make the contract “Caller should pass false only if they will deal with freeing the child”, and so I ended up passing `true` in cases such as here.  I felt like `true` should kind of be the default case, and `false` the exception.

+    /*
+     * The child was pre-existing, so s->old_bs must be non-NULL, and
+     * s->child thus must not have been freed
+     */
+    assert(s->child != NULL);
+    if (!new_bs) {
+        /* As described above, *s->childp was cleared, so restore it */
+        *s->childp = s->child;
+    }
If it wasn't cleared, doesn't it still contain s->child, so this could
just be an unconditional rollback?

I think so, yes.  We’d still have to explain why the rollback is required, though, so would dropping the condition really make it simpler?  (Of course this also goes in the opposite direction: Is making it conditional simpler? :))

I just feel like the comment would be something like “Restore *s->childp in case it was cleared as described above”, and then I’d find it a bit strange if the “in case” isn’t part of the code...  *shrug*


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