From 7248b87cb0292a13c0309a4aba9f5daf7a76d297 Mon Sep 17 00:00:00 2001 From: David Woodhouse Date: Sun, 22 Jan 2023 22:05:37 +0000 Subject: [PATCH] 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 Reviewed-by: Paul Durrant --- hw/i386/kvm/xenstore_impl.c | 150 ++++++++++++++++++++++++++++++++++-- tests/unit/test-xs-node.c | 118 ++++++++++++++++++++++++++++ 2 files changed, 262 insertions(+), 6 deletions(-) diff --git a/hw/i386/kvm/xenstore_impl.c b/hw/i386/kvm/xenstore_impl.c index 9c2348835f..0812e367b0 100644 --- a/hw/i386/kvm/xenstore_impl.c +++ b/hw/i386/kvm/xenstore_impl.c @@ -46,13 +46,56 @@ typedef struct XsWatch { int rel_prefix; } XsWatch; +typedef struct XsTransaction { + XsNode *root; + unsigned int nr_nodes; + unsigned int base_tx; + unsigned int tx_id; + unsigned int dom_id; +} XsTransaction; + struct XenstoreImplState { XsNode *root; unsigned int nr_nodes; GHashTable *watches; unsigned int nr_domu_watches; + GHashTable *transactions; + unsigned int nr_domu_transactions; + unsigned int root_tx; + unsigned int last_tx; }; + +static void nobble_tx(gpointer key, gpointer value, gpointer user_data) +{ + unsigned int *new_tx_id = user_data; + XsTransaction *tx = value; + + if (tx->base_tx == *new_tx_id) { + /* Transactions based on XBT_NULL will always fail */ + tx->base_tx = XBT_NULL; + } +} + +static inline unsigned int next_tx(struct XenstoreImplState *s) +{ + unsigned int tx_id; + + /* Find the next TX id which isn't either XBT_NULL or in use. */ + do { + tx_id = ++s->last_tx; + } while (tx_id == XBT_NULL || tx_id == s->root_tx || + g_hash_table_lookup(s->transactions, GINT_TO_POINTER(tx_id))); + + /* + * It is vanishingly unlikely, but ensure that no outstanding transaction + * is based on the (previous incarnation of the) newly-allocated TX id. + */ + g_hash_table_foreach(s->transactions, nobble_tx, &tx_id); + + return tx_id; +} + static inline XsNode *xs_node_new(void) { XsNode *n = g_new0(XsNode, 1); @@ -159,6 +202,7 @@ struct walk_op { GList *watches; unsigned int dom_id; + unsigned int tx_id; /* The number of nodes which will exist in the tree if this op succeeds. */ unsigned int new_nr_nodes; @@ -176,6 +220,7 @@ struct walk_op { bool inplace; bool mutating; bool create_dirs; + bool in_transaction; }; static void fire_watches(struct walk_op *op, bool parents) @@ -183,7 +228,7 @@ static void fire_watches(struct walk_op *op, bool parents) GList *l = NULL; XsWatch *w; - if (!op->mutating) { + if (!op->mutating || op->in_transaction) { return; } @@ -450,10 +495,23 @@ static int xs_node_walk(XsNode **n, struct walk_op *op) assert(!op->watches); /* * On completing the recursion back up the path walk and reaching the - * top, assign the new node count if the operation was successful. + * top, assign the new node count if the operation was successful. If + * the main tree was changed, bump its tx ID so that outstanding + * transactions correctly fail. But don't bump it every time; only + * if it makes a difference. */ if (!err && op->mutating) { - op->s->nr_nodes = op->new_nr_nodes; + if (!op->in_transaction) { + if (op->s->root_tx != op->s->last_tx) { + op->s->root_tx = next_tx(op->s); + } + op->s->nr_nodes = op->new_nr_nodes; + } else { + XsTransaction *tx = g_hash_table_lookup(op->s->transactions, + GINT_TO_POINTER(op->tx_id)); + assert(tx); + tx->nr_nodes = op->new_nr_nodes; + } } } return err; @@ -535,14 +593,23 @@ static int init_walk_op(XenstoreImplState *s, struct walk_op *op, op->inplace = true; op->mutating = false; op->create_dirs = false; + op->in_transaction = false; op->dom_id = dom_id; + op->tx_id = tx_id; op->s = s; if (tx_id == XBT_NULL) { *rootp = &s->root; op->new_nr_nodes = s->nr_nodes; } else { - return ENOENT; + XsTransaction *tx = g_hash_table_lookup(s->transactions, + GINT_TO_POINTER(tx_id)); + if (!tx) { + return ENOENT; + } + *rootp = &tx->root; + op->new_nr_nodes = tx->nr_nodes; + op->in_transaction = true; } return 0; @@ -616,13 +683,71 @@ int xs_impl_directory(XenstoreImplState *s, unsigned int dom_id, int xs_impl_transaction_start(XenstoreImplState *s, unsigned int dom_id, xs_transaction_t *tx_id) { - return ENOSYS; + XsTransaction *tx; + + if (*tx_id != XBT_NULL) { + return EINVAL; + } + + if (dom_id && s->nr_domu_transactions >= XS_MAX_TRANSACTIONS) { + return ENOSPC; + } + + tx = g_new0(XsTransaction, 1); + + tx->nr_nodes = s->nr_nodes; + tx->tx_id = next_tx(s); + tx->base_tx = s->root_tx; + tx->root = xs_node_ref(s->root); + tx->dom_id = dom_id; + + g_hash_table_insert(s->transactions, GINT_TO_POINTER(tx->tx_id), tx); + if (dom_id) { + s->nr_domu_transactions++; + } + *tx_id = tx->tx_id; + return 0; +} + +static int transaction_commit(XenstoreImplState *s, XsTransaction *tx) +{ + if (s->root_tx != tx->base_tx) { + return EAGAIN; + } + xs_node_unref(s->root); + s->root = tx->root; + tx->root = NULL; + s->root_tx = tx->tx_id; + s->nr_nodes = tx->nr_nodes; + + /* + * XX: Walk the new root and fire watches on any node which has a + * refcount of one (which is therefore unique to this transaction). + */ + return 0; } int xs_impl_transaction_end(XenstoreImplState *s, unsigned int dom_id, xs_transaction_t tx_id, bool commit) { - return ENOSYS; + int ret = 0; + XsTransaction *tx = g_hash_table_lookup(s->transactions, + GINT_TO_POINTER(tx_id)); + + if (!tx || tx->dom_id != dom_id) { + return ENOENT; + } + + if (commit) { + ret = transaction_commit(s, tx); + } + + g_hash_table_remove(s->transactions, GINT_TO_POINTER(tx_id)); + if (dom_id) { + assert(s->nr_domu_transactions); + s->nr_domu_transactions--; + } + return ret; } int xs_impl_rm(XenstoreImplState *s, unsigned int dom_id, @@ -839,15 +964,28 @@ int xs_impl_reset_watches(XenstoreImplState *s, unsigned int dom_id) return 0; } +static void xs_tx_free(void *_tx) +{ + XsTransaction *tx = _tx; + if (tx->root) { + xs_node_unref(tx->root); + } + g_free(tx); +} + XenstoreImplState *xs_impl_create(void) { XenstoreImplState *s = g_new0(XenstoreImplState, 1); s->watches = g_hash_table_new_full(g_str_hash, g_str_equal, g_free, NULL); + s->transactions = g_hash_table_new_full(g_direct_hash, g_direct_equal, + NULL, xs_tx_free); s->nr_nodes = 1; s->root = xs_node_new(); #ifdef XS_NODE_UNIT_TEST s->root->name = g_strdup("/"); #endif + + s->root_tx = s->last_tx = 1; return s; } diff --git a/tests/unit/test-xs-node.c b/tests/unit/test-xs-node.c index 19000b64b2..3c3654550a 100644 --- a/tests/unit/test-xs-node.c +++ b/tests/unit/test-xs-node.c @@ -42,6 +42,7 @@ static void xs_impl_delete(XenstoreImplState *s) g_assert(s->nr_nodes == 1); g_hash_table_unref(s->watches); + g_hash_table_unref(s->transactions); xs_node_unref(s->root); g_free(s); @@ -271,12 +272,129 @@ static void test_xs_node_simple(void) } +static void do_test_xs_node_tx(bool fail, bool commit) +{ + 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/relative/path", + "something"); + g_assert(s->nr_nodes == 7); + g_assert(!err); + g_assert(!strcmp(watches->str, + "some/relative/pathwatch")); + g_string_truncate(watches, 0); + + /* Create a transaction */ + err = xs_impl_transaction_start(s, DOMID_GUEST, &tx_id); + g_assert(!err); + + if (fail) { + /* Write something else in the root */ + err = write_str(s, DOMID_GUEST, XBT_NULL, "some/relative/path", + "another thing"); + g_assert(!err); + g_assert(s->nr_nodes == 7); + g_assert(!strcmp(watches->str, + "some/relative/pathwatch")); + g_string_truncate(watches, 0); + } + + g_assert(!watches->len); + + /* Perform a write in the transaction */ + err = write_str(s, DOMID_GUEST, tx_id, "some/relative/path", + "something else"); + g_assert(!err); + g_assert(s->nr_nodes == 7); + g_assert(!watches->len); + + err = xs_impl_read(s, DOMID_GUEST, XBT_NULL, "some/relative/path", data); + g_assert(!err); + if (fail) { + g_assert(data->len == strlen("another thing")); + g_assert(!memcmp(data->data, "another thing", data->len)); + } else { + g_assert(data->len == strlen("something")); + g_assert(!memcmp(data->data, "something", data->len)); + } + g_byte_array_set_size(data, 0); + + err = xs_impl_read(s, DOMID_GUEST, tx_id, "some/relative/path", data); + g_assert(!err); + g_assert(data->len == strlen("something else")); + g_assert(!memcmp(data->data, "something else", data->len)); + g_byte_array_set_size(data, 0); + + /* Attempt to commit the transaction */ + err = xs_impl_transaction_end(s, DOMID_GUEST, tx_id, commit); + if (commit && fail) { + g_assert(err == EAGAIN); + } else { + g_assert(!err); + } + g_assert(!watches->len); + g_assert(s->nr_nodes == 7); + + err = xs_impl_unwatch(s, DOMID_GUEST, "some", "watch", + watch_cb, watches); + g_assert(!err); + + err = xs_impl_read(s, DOMID_GUEST, XBT_NULL, "some/relative/path", data); + g_assert(!err); + if (fail) { + g_assert(data->len == strlen("another thing")); + g_assert(!memcmp(data->data, "another thing", data->len)); + } else if (commit) { + g_assert(data->len == strlen("something else")); + g_assert(!memcmp(data->data, "something else", data->len)); + } else { + g_assert(data->len == strlen("something")); + g_assert(!memcmp(data->data, "something", data->len)); + } + g_byte_array_unref(data); + g_string_free(watches, true); + xs_impl_delete(s); +} + +static void test_xs_node_tx_fail(void) +{ + do_test_xs_node_tx(true, true); +} + +static void test_xs_node_tx_abort(void) +{ + do_test_xs_node_tx(false, false); + do_test_xs_node_tx(true, false); +} +static void test_xs_node_tx_succeed(void) +{ + do_test_xs_node_tx(false, true); +} + int main(int argc, char **argv) { g_test_init(&argc, &argv, NULL); module_call_init(MODULE_INIT_QOM); g_test_add_func("/xs_node/simple", test_xs_node_simple); + 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); return g_test_run(); }