hw/xen: Implement XenStore transactions
Given that the whole thing supported copy on write from the beginning, transactions end up being fairly simple. On starting a transaction, just take a ref of the existing root; swap it back in on a successful commit. The main tree has a transaction ID too, and we keep a record of the last transaction ID given out. if the main tree is ever modified when it isn't the latest, it gets a new transaction ID. A commit can only succeed if the main tree hasn't moved on since it was forked. Strictly speaking, the XenStore protocol allows a transaction to succeed as long as nothing *it* read or wrote has changed in the interim, but no implementations do that; *any* change is sufficient to abort a transaction. This does not yet fire watches on the changed nodes on a commit. That bit is more fun and will come in a follow-on commit. Signed-off-by: David Woodhouse <dwmw@amazon.co.uk> Reviewed-by: Paul Durrant <paul@xen.org>
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David Woodhouse
parent
6e1330090d
commit
7248b87cb0
@ -46,13 +46,56 @@ typedef struct XsWatch {
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int rel_prefix;
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} XsWatch;
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typedef struct XsTransaction {
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XsNode *root;
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unsigned int nr_nodes;
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unsigned int base_tx;
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unsigned int tx_id;
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unsigned int dom_id;
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} XsTransaction;
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struct XenstoreImplState {
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XsNode *root;
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unsigned int nr_nodes;
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GHashTable *watches;
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unsigned int nr_domu_watches;
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GHashTable *transactions;
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unsigned int nr_domu_transactions;
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unsigned int root_tx;
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unsigned int last_tx;
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};
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static void nobble_tx(gpointer key, gpointer value, gpointer user_data)
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{
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unsigned int *new_tx_id = user_data;
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XsTransaction *tx = value;
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if (tx->base_tx == *new_tx_id) {
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/* Transactions based on XBT_NULL will always fail */
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tx->base_tx = XBT_NULL;
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}
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}
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static inline unsigned int next_tx(struct XenstoreImplState *s)
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{
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unsigned int tx_id;
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/* Find the next TX id which isn't either XBT_NULL or in use. */
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do {
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tx_id = ++s->last_tx;
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} while (tx_id == XBT_NULL || tx_id == s->root_tx ||
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g_hash_table_lookup(s->transactions, GINT_TO_POINTER(tx_id)));
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/*
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* It is vanishingly unlikely, but ensure that no outstanding transaction
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* is based on the (previous incarnation of the) newly-allocated TX id.
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*/
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g_hash_table_foreach(s->transactions, nobble_tx, &tx_id);
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return tx_id;
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}
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static inline XsNode *xs_node_new(void)
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{
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XsNode *n = g_new0(XsNode, 1);
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@ -159,6 +202,7 @@ struct walk_op {
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GList *watches;
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unsigned int dom_id;
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unsigned int tx_id;
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/* The number of nodes which will exist in the tree if this op succeeds. */
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unsigned int new_nr_nodes;
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@ -176,6 +220,7 @@ struct walk_op {
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bool inplace;
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bool mutating;
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bool create_dirs;
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bool in_transaction;
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};
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static void fire_watches(struct walk_op *op, bool parents)
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@ -183,7 +228,7 @@ static void fire_watches(struct walk_op *op, bool parents)
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GList *l = NULL;
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XsWatch *w;
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if (!op->mutating) {
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if (!op->mutating || op->in_transaction) {
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return;
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}
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@ -450,10 +495,23 @@ static int xs_node_walk(XsNode **n, struct walk_op *op)
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assert(!op->watches);
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/*
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* On completing the recursion back up the path walk and reaching the
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* top, assign the new node count if the operation was successful.
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* top, assign the new node count if the operation was successful. If
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* the main tree was changed, bump its tx ID so that outstanding
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* transactions correctly fail. But don't bump it every time; only
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* if it makes a difference.
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*/
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if (!err && op->mutating) {
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if (!op->in_transaction) {
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if (op->s->root_tx != op->s->last_tx) {
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op->s->root_tx = next_tx(op->s);
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}
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op->s->nr_nodes = op->new_nr_nodes;
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} else {
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XsTransaction *tx = g_hash_table_lookup(op->s->transactions,
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GINT_TO_POINTER(op->tx_id));
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assert(tx);
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tx->nr_nodes = op->new_nr_nodes;
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}
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}
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}
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return err;
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@ -535,15 +593,24 @@ static int init_walk_op(XenstoreImplState *s, struct walk_op *op,
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op->inplace = true;
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op->mutating = false;
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op->create_dirs = false;
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op->in_transaction = false;
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op->dom_id = dom_id;
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op->tx_id = tx_id;
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op->s = s;
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if (tx_id == XBT_NULL) {
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*rootp = &s->root;
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op->new_nr_nodes = s->nr_nodes;
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} else {
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XsTransaction *tx = g_hash_table_lookup(s->transactions,
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GINT_TO_POINTER(tx_id));
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if (!tx) {
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return ENOENT;
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}
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*rootp = &tx->root;
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op->new_nr_nodes = tx->nr_nodes;
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op->in_transaction = true;
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}
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return 0;
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}
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@ -616,13 +683,71 @@ int xs_impl_directory(XenstoreImplState *s, unsigned int dom_id,
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int xs_impl_transaction_start(XenstoreImplState *s, unsigned int dom_id,
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xs_transaction_t *tx_id)
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{
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return ENOSYS;
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XsTransaction *tx;
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if (*tx_id != XBT_NULL) {
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return EINVAL;
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}
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if (dom_id && s->nr_domu_transactions >= XS_MAX_TRANSACTIONS) {
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return ENOSPC;
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}
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tx = g_new0(XsTransaction, 1);
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tx->nr_nodes = s->nr_nodes;
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tx->tx_id = next_tx(s);
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tx->base_tx = s->root_tx;
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tx->root = xs_node_ref(s->root);
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tx->dom_id = dom_id;
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g_hash_table_insert(s->transactions, GINT_TO_POINTER(tx->tx_id), tx);
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if (dom_id) {
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s->nr_domu_transactions++;
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}
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*tx_id = tx->tx_id;
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return 0;
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}
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static int transaction_commit(XenstoreImplState *s, XsTransaction *tx)
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{
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if (s->root_tx != tx->base_tx) {
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return EAGAIN;
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}
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xs_node_unref(s->root);
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s->root = tx->root;
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tx->root = NULL;
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s->root_tx = tx->tx_id;
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s->nr_nodes = tx->nr_nodes;
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/*
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* XX: Walk the new root and fire watches on any node which has a
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* refcount of one (which is therefore unique to this transaction).
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*/
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return 0;
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}
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int xs_impl_transaction_end(XenstoreImplState *s, unsigned int dom_id,
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xs_transaction_t tx_id, bool commit)
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{
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return ENOSYS;
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int ret = 0;
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XsTransaction *tx = g_hash_table_lookup(s->transactions,
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GINT_TO_POINTER(tx_id));
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if (!tx || tx->dom_id != dom_id) {
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return ENOENT;
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}
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if (commit) {
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ret = transaction_commit(s, tx);
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}
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g_hash_table_remove(s->transactions, GINT_TO_POINTER(tx_id));
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if (dom_id) {
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assert(s->nr_domu_transactions);
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s->nr_domu_transactions--;
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}
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return ret;
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}
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int xs_impl_rm(XenstoreImplState *s, unsigned int dom_id,
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@ -839,15 +964,28 @@ int xs_impl_reset_watches(XenstoreImplState *s, unsigned int dom_id)
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return 0;
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}
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static void xs_tx_free(void *_tx)
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{
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XsTransaction *tx = _tx;
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if (tx->root) {
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xs_node_unref(tx->root);
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}
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g_free(tx);
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}
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XenstoreImplState *xs_impl_create(void)
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{
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XenstoreImplState *s = g_new0(XenstoreImplState, 1);
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s->watches = g_hash_table_new_full(g_str_hash, g_str_equal, g_free, NULL);
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s->transactions = g_hash_table_new_full(g_direct_hash, g_direct_equal,
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NULL, xs_tx_free);
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s->nr_nodes = 1;
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s->root = xs_node_new();
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#ifdef XS_NODE_UNIT_TEST
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s->root->name = g_strdup("/");
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#endif
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s->root_tx = s->last_tx = 1;
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return s;
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}
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@ -42,6 +42,7 @@ static void xs_impl_delete(XenstoreImplState *s)
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g_assert(s->nr_nodes == 1);
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g_hash_table_unref(s->watches);
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g_hash_table_unref(s->transactions);
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xs_node_unref(s->root);
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g_free(s);
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@ -271,12 +272,129 @@ static void test_xs_node_simple(void)
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}
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static void do_test_xs_node_tx(bool fail, bool commit)
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{
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XenstoreImplState *s = setup();
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GString *watches = g_string_new(NULL);
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GByteArray *data = g_byte_array_new();
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unsigned int tx_id = XBT_NULL;
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int err;
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g_assert(s);
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/* Set a watch */
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err = xs_impl_watch(s, DOMID_GUEST, "some", "watch",
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watch_cb, watches);
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g_assert(!err);
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g_assert(watches->len == strlen("somewatch"));
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g_assert(!strcmp(watches->str, "somewatch"));
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g_string_truncate(watches, 0);
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/* Write something */
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err = write_str(s, DOMID_GUEST, XBT_NULL, "some/relative/path",
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"something");
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g_assert(s->nr_nodes == 7);
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g_assert(!err);
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g_assert(!strcmp(watches->str,
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"some/relative/pathwatch"));
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g_string_truncate(watches, 0);
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/* Create a transaction */
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err = xs_impl_transaction_start(s, DOMID_GUEST, &tx_id);
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g_assert(!err);
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if (fail) {
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/* Write something else in the root */
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err = write_str(s, DOMID_GUEST, XBT_NULL, "some/relative/path",
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"another thing");
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g_assert(!err);
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g_assert(s->nr_nodes == 7);
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g_assert(!strcmp(watches->str,
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"some/relative/pathwatch"));
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g_string_truncate(watches, 0);
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}
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g_assert(!watches->len);
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/* Perform a write in the transaction */
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err = write_str(s, DOMID_GUEST, tx_id, "some/relative/path",
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"something else");
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g_assert(!err);
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g_assert(s->nr_nodes == 7);
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g_assert(!watches->len);
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err = xs_impl_read(s, DOMID_GUEST, XBT_NULL, "some/relative/path", data);
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g_assert(!err);
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if (fail) {
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g_assert(data->len == strlen("another thing"));
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g_assert(!memcmp(data->data, "another thing", data->len));
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} else {
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g_assert(data->len == strlen("something"));
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g_assert(!memcmp(data->data, "something", data->len));
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}
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g_byte_array_set_size(data, 0);
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err = xs_impl_read(s, DOMID_GUEST, tx_id, "some/relative/path", data);
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g_assert(!err);
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g_assert(data->len == strlen("something else"));
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g_assert(!memcmp(data->data, "something else", data->len));
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g_byte_array_set_size(data, 0);
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/* Attempt to commit the transaction */
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err = xs_impl_transaction_end(s, DOMID_GUEST, tx_id, commit);
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if (commit && fail) {
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g_assert(err == EAGAIN);
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} else {
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g_assert(!err);
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}
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g_assert(!watches->len);
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g_assert(s->nr_nodes == 7);
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err = xs_impl_unwatch(s, DOMID_GUEST, "some", "watch",
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watch_cb, watches);
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g_assert(!err);
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err = xs_impl_read(s, DOMID_GUEST, XBT_NULL, "some/relative/path", data);
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g_assert(!err);
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if (fail) {
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g_assert(data->len == strlen("another thing"));
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g_assert(!memcmp(data->data, "another thing", data->len));
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} else if (commit) {
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g_assert(data->len == strlen("something else"));
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g_assert(!memcmp(data->data, "something else", data->len));
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} else {
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g_assert(data->len == strlen("something"));
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g_assert(!memcmp(data->data, "something", data->len));
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}
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g_byte_array_unref(data);
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g_string_free(watches, true);
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xs_impl_delete(s);
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}
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static void test_xs_node_tx_fail(void)
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{
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do_test_xs_node_tx(true, true);
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}
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static void test_xs_node_tx_abort(void)
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{
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do_test_xs_node_tx(false, false);
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do_test_xs_node_tx(true, false);
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}
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static void test_xs_node_tx_succeed(void)
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{
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do_test_xs_node_tx(false, true);
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}
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int main(int argc, char **argv)
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{
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g_test_init(&argc, &argv, NULL);
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module_call_init(MODULE_INIT_QOM);
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g_test_add_func("/xs_node/simple", test_xs_node_simple);
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g_test_add_func("/xs_node/tx_abort", test_xs_node_tx_abort);
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g_test_add_func("/xs_node/tx_fail", test_xs_node_tx_fail);
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g_test_add_func("/xs_node/tx_succeed", test_xs_node_tx_succeed);
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return g_test_run();
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}
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