| Message ID | 20211111120829.81329-1-hreitz@redhat.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | block: Attempt on fixing 030-reported errors | expand |
Am 11.11.2021 um 13:08 hat Hanna Reitz geschrieben: > 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. Thanks, applied to the block branch. Kevin
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(-)