block: Let replace_child_tran keep indirect pointer
As of a future commit, bdrv_replace_child_noperm() will clear the
indirect BdrvChild pointer passed to it if the new child BDS is NULL.
bdrv_replace_child_tran() will want to let it do that, but revert this
change in its abort handler. For that, we need to have it receive a
BdrvChild ** pointer, too, and keep it stored in the
BdrvReplaceChildState object that we attach to the transaction.
Note that we do not need to store it in the BdrvReplaceChildState when
new_bs is not NULL, because then there is nothing to revert. This is
important so that bdrv_replace_node_noperm() can pass a pointer to a
loop-local variable to bdrv_replace_child_tran() without worrying that
this pointer will outlive one loop iteration.
(Of course, for that to work, bdrv_replace_node_noperm() and in turn
bdrv_replace_node() and its relatives may not be called with a NULL @to
node. Luckily, they already are not, but now we should assert this.)
bdrv_remove_file_or_backing_child() on the other hand needs to ensure
that the indirect pointer it passes will stay valid for the duration of
the transaction. Ensure this by keeping a strong reference to the BDS
whose &bs->backing or &bs->file it passes to bdrv_replace_child_tran(),
and giving up that reference only in the transaction .clean() handler.
Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20211111120829.81329-9-hreitz@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20211115145409.176785-9-kwolf@redhat.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>
diff --git a/block.c b/block.c
index 8da057f..a400271 100644
--- a/block.c
+++ b/block.c
@@ -2254,6 +2254,7 @@
typedef struct BdrvReplaceChildState {
BdrvChild *child;
+ BdrvChild **childp;
BlockDriverState *old_bs;
} BdrvReplaceChildState;
@@ -2269,7 +2270,29 @@
BdrvReplaceChildState *s = opaque;
BlockDriverState *new_bs = s->child->bs;
- /* old_bs reference is transparently moved from @s to @s->child */
+ /*
+ * old_bs reference is transparently moved from @s to s->child.
+ *
+ * Pass &s->child here instead of s->childp, because:
+ * (1) s->old_bs must be non-NULL, so bdrv_replace_child_noperm() will not
+ * modify the BdrvChild * pointer we indirectly pass to it, i.e. it
+ * will not modify s->child. From that perspective, it does not matter
+ * whether we pass s->childp or &s->child.
+ * (TODO: Right now, bdrv_replace_child_noperm() never modifies that
+ * pointer anyway (though it will in the future), so at this point it
+ * absolutely does not matter whether we pass s->childp or &s->child.)
+ * (2) If new_bs is not NULL, s->childp will be NULL. We then cannot use
+ * it here.
+ * (3) If new_bs is NULL, *s->childp will have been NULLed by
+ * bdrv_replace_child_tran()'s bdrv_replace_child_noperm() call, and we
+ * must not pass a NULL *s->childp here.
+ * (TODO: In its current state, bdrv_replace_child_noperm() will not
+ * have NULLed *s->childp, so this does not apply yet. It will in the
+ * future.)
+ *
+ * So whether new_bs was NULL or not, we cannot pass s->childp here; and in
+ * any case, there is no reason to pass it anyway.
+ */
bdrv_replace_child_noperm(&s->child, s->old_bs);
bdrv_unref(new_bs);
}
@@ -2286,22 +2309,32 @@
* Note: real unref of old_bs is done only on commit.
*
* The function doesn't update permissions, caller is responsible for this.
+ *
+ * Note that if new_bs == NULL, @childp is stored in a state object attached
+ * to @tran, so that the old child can be reinstated in the abort handler.
+ * Therefore, if @new_bs can be NULL, @childp must stay valid until the
+ * transaction is committed or aborted.
+ *
+ * (TODO: The reinstating does not happen yet, but it will once
+ * bdrv_replace_child_noperm() NULLs *childp when new_bs is NULL.)
*/
-static void bdrv_replace_child_tran(BdrvChild *child, BlockDriverState *new_bs,
+static void bdrv_replace_child_tran(BdrvChild **childp,
+ BlockDriverState *new_bs,
Transaction *tran)
{
BdrvReplaceChildState *s = g_new(BdrvReplaceChildState, 1);
*s = (BdrvReplaceChildState) {
- .child = child,
- .old_bs = child->bs,
+ .child = *childp,
+ .childp = new_bs == NULL ? childp : NULL,
+ .old_bs = (*childp)->bs,
};
tran_add(tran, &bdrv_replace_child_drv, s);
if (new_bs) {
bdrv_ref(new_bs);
}
- bdrv_replace_child_noperm(&child, new_bs);
- /* old_bs reference is transparently moved from @child to @s */
+ bdrv_replace_child_noperm(childp, new_bs);
+ /* old_bs reference is transparently moved from *childp to @s */
}
/*
@@ -4844,6 +4877,7 @@
typedef struct BdrvRemoveFilterOrCowChild {
BdrvChild *child;
+ BlockDriverState *bs;
bool is_backing;
} BdrvRemoveFilterOrCowChild;
@@ -4873,10 +4907,19 @@
bdrv_child_free(s->child);
}
+static void bdrv_remove_filter_or_cow_child_clean(void *opaque)
+{
+ BdrvRemoveFilterOrCowChild *s = opaque;
+
+ /* Drop the bs reference after the transaction is done */
+ bdrv_unref(s->bs);
+ g_free(s);
+}
+
static TransactionActionDrv bdrv_remove_filter_or_cow_child_drv = {
.abort = bdrv_remove_filter_or_cow_child_abort,
.commit = bdrv_remove_filter_or_cow_child_commit,
- .clean = g_free,
+ .clean = bdrv_remove_filter_or_cow_child_clean,
};
/*
@@ -4894,6 +4937,11 @@
return;
}
+ /*
+ * Keep a reference to @bs so @childp will stay valid throughout the
+ * transaction (required by bdrv_replace_child_tran())
+ */
+ bdrv_ref(bs);
if (child == bs->backing) {
childp = &bs->backing;
} else if (child == bs->file) {
@@ -4903,12 +4951,13 @@
}
if (child->bs) {
- bdrv_replace_child_tran(*childp, NULL, tran);
+ bdrv_replace_child_tran(childp, NULL, tran);
}
s = g_new(BdrvRemoveFilterOrCowChild, 1);
*s = (BdrvRemoveFilterOrCowChild) {
.child = child,
+ .bs = bs,
.is_backing = (childp == &bs->backing),
};
tran_add(tran, &bdrv_remove_filter_or_cow_child_drv, s);
@@ -4934,6 +4983,8 @@
{
BdrvChild *c, *next;
+ assert(to != NULL);
+
QLIST_FOREACH_SAFE(c, &from->parents, next_parent, next) {
assert(c->bs == from);
if (!should_update_child(c, to)) {
@@ -4949,7 +5000,12 @@
c->name, from->node_name);
return -EPERM;
}
- bdrv_replace_child_tran(c, to, tran);
+
+ /*
+ * Passing a pointer to the local variable @c is fine here, because
+ * @to is not NULL, and so &c will not be attached to the transaction.
+ */
+ bdrv_replace_child_tran(&c, to, tran);
}
return 0;
@@ -4964,6 +5020,8 @@
*
* With @detach_subchain=true @to must be in a backing chain of @from. In this
* case backing link of the cow-parent of @to is removed.
+ *
+ * @to must not be NULL.
*/
static int bdrv_replace_node_common(BlockDriverState *from,
BlockDriverState *to,
@@ -4976,6 +5034,8 @@
BlockDriverState *to_cow_parent = NULL;
int ret;
+ assert(to != NULL);
+
if (detach_subchain) {
assert(bdrv_chain_contains(from, to));
assert(from != to);
@@ -5031,6 +5091,9 @@
return ret;
}
+/**
+ * Replace node @from by @to (where neither may be NULL).
+ */
int bdrv_replace_node(BlockDriverState *from, BlockDriverState *to,
Error **errp)
{
@@ -5098,7 +5161,7 @@
bdrv_drained_begin(old_bs);
bdrv_drained_begin(new_bs);
- bdrv_replace_child_tran(child, new_bs, tran);
+ bdrv_replace_child_tran(&child, new_bs, tran);
found = g_hash_table_new(NULL, NULL);
refresh_list = bdrv_topological_dfs(refresh_list, found, old_bs);
