Hi,
v1 cover letter:
https://lists.nongnu.org/archive/html/qemu-devel/2021-11/msg01287.html
In v2 I’ve addressed the comments I’ve received from Kevin and Vladimir.
To this end, I’ve retained only the non-controversial part in patch 5,
and split everything else (i.e. the stuff relating to
bdrv_replace_child_tran()) into the dedicated patches 6 and 8.
Kevin’s most important comments (to my understanding) were that:
(A) When bdrv_remove_file_or_backing_child() uses
bdrv_replace_child_tran(), we have to ensure that the BDS lives as
long as the transaction does. This is solved by keeping a strong
reference to it that’s released only when the transaction is cleaned
up (and the new patch 7 ensures that the .clean() handler will be
invoked after all .commit()/.abort() handlers, so the reference will
really live as long as the transaction does).
(B) bdrv_replace_node_noperm() passes a pointer to loop-local variable,
which is a really bad idea considering the transaction lives much
longer than one loop iteration.
Luckily, the new_bs it passes is always non-NULL, and so
bdrv_replace_child_tran() actually doesn’t need to store the
BdrvChild ** pointer, because for a non-NULL new_bs, there is
nothing to revert in the abort handler. We just need to clarify
this, not store the pointer in case of a non-NULL new_bs, and assert
that bdrv_replace_node_noperm() and its relatives are only used to
replace an existing node by some other existing (i.e. non-NULL)
node.
git-backport-diff against v1:
Key:
[----] : patches are identical
[####] : number of functional differences between upstream/downstream patch
[down] : patch is downstream-only
The flags [FC] indicate (F)unctional and (C)ontextual differences, respectively
001/10:[----] [--] 'stream: Traverse graph after modification'
002/10:[0005] [FC] 'block: Manipulate children list in .attach/.detach'
003/10:[----] [--] 'block: Unite remove_empty_child and child_free'
004/10:[----] [--] 'block: Drop detached child from ignore list'
005/10:[0040] [FC] 'block: Pass BdrvChild ** to replace_child_noperm'
006/10:[down] 'block: Restructure remove_file_or_backing_child()'
007/10:[down] 'transactions: Invoke clean() after everything else'
008/10:[down] 'block: Let replace_child_tran keep indirect pointer'
009/10:[0020] [FC] 'block: Let replace_child_noperm free children'
010/10:[----] [--] 'iotests/030: Unthrottle parallel jobs in reverse'
Detailed per-patch changes:
Patch 2:
- Dropped stale comment about undoing bdrv_attach_child_noperm()’s list
insertion in the respective abort handler
Patch 5:
- Split off everything related to bdrv_replace_child_tran()
Patch 6:
- Added (split off from patch 5)
- Keep the `if (!child) { return; }` path before getting childp
Patch 7:
- Added: I wanted bdrv_remove_file_or_backing_child() to keep a strong
BDS reference throughout the transaction, and the simplest way (i.e.
the one where I wouldn’t have to think too hard) was to add this
patch and have transactions invoke .clean() after all
.commit()/.abort() handlers are done. This way we can just drop the
reference in .clean() and need not care about the order the
transaction actions were added in.
Patch 8:
- Added (split off from patch 5)
- The BdrvChild ** pointer is now only stored in the
BdrvReplaceChildState if new_bs is NULL. Otherwise, we don’t need
it, because we won’t need to reinstate the child in the abort
handler. This way we don’t have to worry about
bdrv_replace_node_noperm() passing a pointer to a loop-local
variable (because the new_bs it passes is never NULL).
- In the same vein, assert that bdrv_replace_node() and relatives
cannot be called to replace the node by NULL.
- Have bdrv_remove_file_or_backing_child() keep a strong reference to
the parent BDS throughout the transaction, so the &bs->backing or
&bs->file pointer remains valid
- Moved the comment explaining why we want to pass &s->child instead of
s->childp to bdrv_replace_child_noperm() in
bdrv_replace_child_abort() from patch 9 here (and also keep passing
&s->child instead of s->childp). It is already relevant now that
s->childp is valid only if new_bs is NULL.
Patch 9:
- The comment this used to add to bdrv_replace_child_abort() is now
already added by patch 8, we just need to drop the TODO note there
- Also drop the TODO note above bdrv_replace_child_tran()
Hanna Reitz (10):
stream: Traverse graph after modification
block: Manipulate children list in .attach/.detach
block: Unite remove_empty_child and child_free
block: Drop detached child from ignore list
block: Pass BdrvChild ** to replace_child_noperm
block: Restructure remove_file_or_backing_child()
transactions: Invoke clean() after everything else
block: Let replace_child_tran keep indirect pointer
block: Let replace_child_noperm free children
iotests/030: Unthrottle parallel jobs in reverse
include/qemu/transactions.h | 3 +
block.c | 233 +++++++++++++++++++++++++++---------
block/stream.c | 7 +-
util/transactions.c | 8 +-
tests/qemu-iotests/030 | 11 +-
5 files changed, 201 insertions(+), 61 deletions(-)
--
2.33.1
