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hw/xen: Watches on XenStore transactions 

Firing watches on the nodes that still exist is relatively easy; just
walk the tree and look at the nodes with refcount of one.

Firing watches on *deleted* nodes is more fun. We add 'modified_in_tx'
and 'deleted_in_tx' flags to each node. Nodes with those flags cannot
be shared, as they will always be unique to the transaction in which
they were created.

When xs_node_walk would need to *create* a node as scaffolding and it
encounters a deleted_in_tx node, it can resurrect it simply by clearing
its deleted_in_tx flag. If that node originally had any *data*, they're
gone, and the modified_in_tx flag will have been set when it was first
deleted.

We then attempt to send appropriate watches when the transaction is
committed, properly delete the deleted_in_tx nodes, and remove the
modified_in_tx flag from the others.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
This commit is contained in:
David Woodhouse 2023-01-22 22:59:49 +00:00
parent 7248b87cb0
commit 7cabbdb70d
2 changed files with 380 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

151
hw/i386/kvm/xenstore_impl.c
View File

@ -32,6 +32,8 @@ typedef struct XsNode {
GByteArray *content;
GHashTable *children;
uint64_t gencnt;
bool deleted_in_tx;
bool modified_in_tx;
#ifdef XS_NODE_UNIT_TEST
gchar *name; /* debug only */
#endif
@ -153,6 +155,13 @@ static XsNode *xs_node_copy(XsNode *old)
XsNode *n = xs_node_new();
n->gencnt = old->gencnt;
#ifdef XS_NODE_UNIT_TEST
if (n->name) {
n->name = g_strdup(old->name);
}
#endif
if (old->children) {
n->children = g_hash_table_new_full(g_str_hash, g_str_equal, g_free,
(GDestroyNotify)xs_node_unref);
@ -221,6 +230,9 @@ struct walk_op {
bool mutating;
bool create_dirs;
bool in_transaction;
/* Tracking during recursion so we know which is first. */
bool deleted_in_tx;
};
static void fire_watches(struct walk_op *op, bool parents)
@ -277,6 +289,9 @@ static int xs_node_add_content(XsNode **n, struct walk_op *op)
g_byte_array_unref((*n)->content);
}
(*n)->content = g_byte_array_ref(data);
if (op->tx_id != XBT_NULL) {
(*n)->modified_in_tx = true;
}
return 0;
}
@ -333,10 +348,62 @@ static int node_rm_recurse(gpointer key, gpointer value, gpointer user_data)
return this_inplace;
}
static XsNode *xs_node_copy_deleted(XsNode *old, struct walk_op *op);
static void copy_deleted_recurse(gpointer key, gpointer value,
gpointer user_data)
{
struct walk_op *op = user_data;
GHashTable *siblings = op->op_opaque2;
XsNode *n = xs_node_copy_deleted(value, op);
/*
* Reinsert the deleted_in_tx copy of the node into the parent's
* 'children' hash table. Having stashed it from op->op_opaque2
* before the recursive call to xs_node_copy_deleted() scribbled
* over it.
*/
g_hash_table_insert(siblings, g_strdup(key), n);
}
static XsNode *xs_node_copy_deleted(XsNode *old, struct walk_op *op)
{
XsNode *n = xs_node_new();
n->gencnt = old->gencnt;
#ifdef XS_NODE_UNIT_TEST
if (old->name) {
n->name = g_strdup(old->name);
}
#endif
if (old->children) {
n->children = g_hash_table_new_full(g_str_hash, g_str_equal, g_free,
(GDestroyNotify)xs_node_unref);
op->op_opaque2 = n->children;
g_hash_table_foreach(old->children, copy_deleted_recurse, op);
}
n->deleted_in_tx = true;
/* If it gets resurrected we only fire a watch if it lost its content */
if (old->content) {
n->modified_in_tx = true;
}
op->new_nr_nodes--;
return n;
}
static int xs_node_rm(XsNode **n, struct walk_op *op)
{
bool this_inplace = op->inplace;
if (op->tx_id != XBT_NULL) {
/* It's not trivial to do inplace handling for this one */
XsNode *old = *n;
*n = xs_node_copy_deleted(old, op);
xs_node_unref(old);
return 0;
}
/* Fire watches for, and count, nodes in the subtree which get deleted */
if ((*n)->children) {
g_hash_table_foreach_remove((*n)->children, node_rm_recurse, op);
@ -408,6 +475,10 @@ static int xs_node_walk(XsNode **n, struct walk_op *op)
}
if (child) {
if (child->deleted_in_tx) {
assert(child->ref == 1);
/* Cannot actually set child->deleted_in_tx = false until later */
}
xs_node_ref(child);
/*
* Now we own it too. But if we can modify inplace, that's going to
@ -475,6 +546,15 @@ static int xs_node_walk(XsNode **n, struct walk_op *op)
xs_node_unref(old);
}
/*
* If we resurrected a deleted_in_tx node, we can mark it as no longer
* deleted now that we know the overall operation has succeeded.
*/
if (op->create_dirs && child && child->deleted_in_tx) {
op->new_nr_nodes++;
child->deleted_in_tx = false;
}
/*
* The child may be NULL here, for a remove operation. Either way,
* xs_node_add_child() will do the right thing and return a value
@ -709,8 +789,69 @@ int xs_impl_transaction_start(XenstoreImplState *s, unsigned int dom_id,
return 0;
}
static gboolean tx_commit_walk(gpointer key, gpointer value,
gpointer user_data)
{
struct walk_op *op = user_data;
int path_len = strlen(op->path);
int key_len = strlen(key);
bool fire_parents = true;
XsWatch *watch;
XsNode *n = value;
if (n->ref != 1) {
return false;
}
if (n->deleted_in_tx) {
/*
* We fire watches on our parents if we are the *first* node
* to be deleted (the topmost one). This matches the behaviour
* when deleting in the live tree.
*/
fire_parents = !op->deleted_in_tx;
/* Only used on the way down so no need to clear it later */
op->deleted_in_tx = true;
}
assert(key_len + path_len + 2 <= sizeof(op->path));
op->path[path_len] = '/';
memcpy(op->path + path_len + 1, key, key_len + 1);
watch = g_hash_table_lookup(op->s->watches, op->path);
if (watch) {
op->watches = g_list_append(op->watches, watch);
}
if (n->children) {
g_hash_table_foreach_remove(n->children, tx_commit_walk, op);
}
if (watch) {
op->watches = g_list_remove(op->watches, watch);
}
/*
* Don't fire watches if this node was only copied because a
* descendent was changed. The modified_in_tx flag indicates the
* ones which were really changed.
*/
if (n->modified_in_tx || n->deleted_in_tx) {
fire_watches(op, fire_parents);
n->modified_in_tx = false;
}
op->path[path_len] = '\0';
/* Deleted nodes really do get expunged when we commit */
return n->deleted_in_tx;
}
static int transaction_commit(XenstoreImplState *s, XsTransaction *tx)
{
struct walk_op op;
XsNode **n;
if (s->root_tx != tx->base_tx) {
return EAGAIN;
}
@ -720,10 +861,18 @@ static int transaction_commit(XenstoreImplState *s, XsTransaction *tx)
s->root_tx = tx->tx_id;
s->nr_nodes = tx->nr_nodes;
init_walk_op(s, &op, XBT_NULL, tx->dom_id, "/", &n);
op.deleted_in_tx = false;
op.mutating = true;
/*
* XX: Walk the new root and fire watches on any node which has a
* Walk the new root and fire watches on any node which has a
* refcount of one (which is therefore unique to this transaction).
*/
if (s->root->children) {
g_hash_table_foreach_remove(s->root->children, tx_commit_walk, &op);
}
return 0;
}

231
tests/unit/test-xs-node.c
View File

@ -347,7 +347,13 @@ static void do_test_xs_node_tx(bool fail, bool commit)
} else {
g_assert(!err);
}
g_assert(!watches->len);
if (commit && !fail) {
g_assert(!strcmp(watches->str,
"some/relative/pathwatch"));
g_string_truncate(watches, 0);
} else {
g_assert(!watches->len);
}
g_assert(s->nr_nodes == 7);
err = xs_impl_unwatch(s, DOMID_GUEST, "some", "watch",
@ -386,6 +392,226 @@ static void test_xs_node_tx_succeed(void)
do_test_xs_node_tx(false, true);
}
static void test_xs_node_tx_rm(void)
{
XenstoreImplState *s = setup();
GString *watches = g_string_new(NULL);
GByteArray *data = g_byte_array_new();
unsigned int tx_id = XBT_NULL;
int err;
g_assert(s);
/* Set a watch */
err = xs_impl_watch(s, DOMID_GUEST, "some", "watch",
watch_cb, watches);
g_assert(!err);
g_assert(watches->len == strlen("somewatch"));
g_assert(!strcmp(watches->str, "somewatch"));
g_string_truncate(watches, 0);
/* Write something */
err = write_str(s, DOMID_GUEST, XBT_NULL, "some/deep/dark/relative/path",
"something");
g_assert(!err);
g_assert(s->nr_nodes == 9);
g_assert(!strcmp(watches->str,
"some/deep/dark/relative/pathwatch"));
g_string_truncate(watches, 0);
/* Create a transaction */
err = xs_impl_transaction_start(s, DOMID_GUEST, &tx_id);
g_assert(!err);
/* Delete the tree in the transaction */
err = xs_impl_rm(s, DOMID_GUEST, tx_id, "some/deep/dark");
g_assert(!err);
g_assert(s->nr_nodes == 9);
g_assert(!watches->len);
err = xs_impl_read(s, DOMID_GUEST, XBT_NULL, "some/deep/dark/relative/path",
data);
g_assert(!err);
g_assert(data->len == strlen("something"));
g_assert(!memcmp(data->data, "something", data->len));
g_byte_array_set_size(data, 0);
/* Commit the transaction */
err = xs_impl_transaction_end(s, DOMID_GUEST, tx_id, true);
g_assert(!err);
g_assert(s->nr_nodes == 6);
g_assert(!strcmp(watches->str, "some/deep/darkwatch"));
g_string_truncate(watches, 0);
/* Now the node is gone */
err = xs_impl_read(s, DOMID_GUEST, XBT_NULL, "some/deep/dark/relative/path",
data);
g_assert(err == ENOENT);
g_byte_array_unref(data);
err = xs_impl_unwatch(s, DOMID_GUEST, "some", "watch",
watch_cb, watches);
g_assert(!err);
g_string_free(watches, true);
xs_impl_delete(s);
}
static void test_xs_node_tx_resurrect(void)
{
XenstoreImplState *s = setup();
GString *watches = g_string_new(NULL);
GByteArray *data = g_byte_array_new();
unsigned int tx_id = XBT_NULL;
int err;
g_assert(s);
/* Write something */
err = write_str(s, DOMID_GUEST, XBT_NULL, "some/deep/dark/relative/path",
"something");
g_assert(!err);
g_assert(s->nr_nodes == 9);
/* This node will be wiped and resurrected */
err = write_str(s, DOMID_GUEST, XBT_NULL, "some/deep/dark",
"foo");
g_assert(!err);
g_assert(s->nr_nodes == 9);
/* Set a watch */
err = xs_impl_watch(s, DOMID_GUEST, "some", "watch",
watch_cb, watches);
g_assert(!err);
g_assert(watches->len == strlen("somewatch"));
g_assert(!strcmp(watches->str, "somewatch"));
g_string_truncate(watches, 0);
/* Create a transaction */
err = xs_impl_transaction_start(s, DOMID_GUEST, &tx_id);
g_assert(!err);
/* Delete the tree in the transaction */
err = xs_impl_rm(s, DOMID_GUEST, tx_id, "some/deep");
g_assert(!err);
g_assert(s->nr_nodes == 9);
g_assert(!watches->len);
/* Resurrect part of it */
err = write_str(s, DOMID_GUEST, tx_id, "some/deep/dark/different/path",
"something");
g_assert(!err);
g_assert(s->nr_nodes == 9);
/* Commit the transaction */
err = xs_impl_transaction_end(s, DOMID_GUEST, tx_id, true);
g_assert(!err);
g_assert(s->nr_nodes == 9);
/* lost data */
g_assert(strstr(watches->str, "some/deep/dark/different/pathwatch"));
/* topmost deleted */
g_assert(strstr(watches->str, "some/deep/dark/relativewatch"));
/* lost data */
g_assert(strstr(watches->str, "some/deep/darkwatch"));
g_string_truncate(watches, 0);
/* Now the node is gone */
err = xs_impl_read(s, DOMID_GUEST, XBT_NULL, "some/deep/dark/relative/path",
data);
g_assert(err == ENOENT);
g_byte_array_unref(data);
err = xs_impl_unwatch(s, DOMID_GUEST, "some", "watch",
watch_cb, watches);
g_assert(!err);
g_string_free(watches, true);
xs_impl_delete(s);
}
static void test_xs_node_tx_resurrect2(void)
{
XenstoreImplState *s = setup();
GString *watches = g_string_new(NULL);
GByteArray *data = g_byte_array_new();
unsigned int tx_id = XBT_NULL;
int err;
g_assert(s);
/* Write something */
err = write_str(s, DOMID_GUEST, XBT_NULL, "some/deep/dark/relative/path",
"something");
g_assert(!err);
g_assert(s->nr_nodes == 9);
/* Another node to remain shared */
err = write_str(s, DOMID_GUEST, XBT_NULL, "some/place/safe", "keepme");
g_assert(!err);
g_assert(s->nr_nodes == 11);
/* This node will be wiped and resurrected */
err = write_str(s, DOMID_GUEST, XBT_NULL, "some/deep/dark",
"foo");
g_assert(!err);
g_assert(s->nr_nodes == 11);
/* Set a watch */
err = xs_impl_watch(s, DOMID_GUEST, "some", "watch",
watch_cb, watches);
g_assert(!err);
g_assert(watches->len == strlen("somewatch"));
g_assert(!strcmp(watches->str, "somewatch"));
g_string_truncate(watches, 0);
/* Create a transaction */
err = xs_impl_transaction_start(s, DOMID_GUEST, &tx_id);
g_assert(!err);
/* Delete the tree in the transaction */
err = xs_impl_rm(s, DOMID_GUEST, tx_id, "some/deep");
g_assert(!err);
g_assert(s->nr_nodes == 11);
g_assert(!watches->len);
/* Resurrect part of it */
err = write_str(s, DOMID_GUEST, tx_id, "some/deep/dark/relative/path",
"something");
g_assert(!err);
g_assert(s->nr_nodes == 11);
/* Commit the transaction */
err = xs_impl_transaction_end(s, DOMID_GUEST, tx_id, true);
g_assert(!err);
g_assert(s->nr_nodes == 11);
/* lost data */
g_assert(strstr(watches->str, "some/deep/dark/relative/pathwatch"));
/* lost data */
g_assert(strstr(watches->str, "some/deep/darkwatch"));
g_string_truncate(watches, 0);
/* Now the node is gone */
err = xs_impl_read(s, DOMID_GUEST, XBT_NULL, "some/deep/dark/relative/path",
data);
g_assert(!err);
g_assert(data->len == strlen("something"));
g_assert(!memcmp(data->data, "something", data->len));
g_byte_array_unref(data);
err = xs_impl_unwatch(s, DOMID_GUEST, "some", "watch",
watch_cb, watches);
g_assert(!err);
g_string_free(watches, true);
xs_impl_delete(s);
}
int main(int argc, char **argv)
{
g_test_init(&argc, &argv, NULL);
@ -395,6 +621,9 @@ int main(int argc, char **argv)
g_test_add_func("/xs_node/tx_abort", test_xs_node_tx_abort);
g_test_add_func("/xs_node/tx_fail", test_xs_node_tx_fail);
g_test_add_func("/xs_node/tx_succeed", test_xs_node_tx_succeed);
g_test_add_func("/xs_node/tx_rm", test_xs_node_tx_rm);
g_test_add_func("/xs_node/tx_resurrect", test_xs_node_tx_resurrect);
g_test_add_func("/xs_node/tx_resurrect2", test_xs_node_tx_resurrect2);
return g_test_run();
}