Merge pull request #2337 from wiredtiger/WT-2241

WT-2241 Use a lock to protect transaction ID allocation.

(cherry picked from commit 6c7338f2e62d74d59d590a6712eb7e55f2586a8a)

3 files changed, 13 insertions, 15 deletions

diff --git a/src/include/txn.h b/src/include/txn.h
index 634363b9216..9f9f282bfa4 100644
--- a/src/include/txn.h
+++ b/src/include/txn.h
@@ -34,7 +34,7 @@ struct WT_COMPILER_TYPE_ALIGN(WT_CACHE_LINE_ALIGNMENT) __wt_txn_state {
 };
 
 struct __wt_txn_global {
-	uint64_t alloc;			/* Transaction ID to allocate. */
+	WT_SPINLOCK id_lock;
 	volatile uint64_t current;	/* Current transaction ID. */
 
 	/* The oldest running transaction ID (may race). */
diff --git a/src/include/txn.i b/src/include/txn.i
index ceea2e4b581..d7d958e801e 100644
--- a/src/include/txn.i
+++ b/src/include/txn.i
@@ -314,7 +314,6 @@ __wt_txn_id_alloc(WT_SESSION_IMPL *session, bool publish)
 {
 	WT_TXN_GLOBAL *txn_global;
 	uint64_t id;
-	u_int i;
 
 	txn_global = &S2C(session)->txn_global;
 
@@ -341,20 +340,16 @@ __wt_txn_id_alloc(WT_SESSION_IMPL *session, bool publish)
 	 * global current ID, so we want post-increment semantics.  Our atomic
 	 * add primitive does pre-increment, so adjust the result here.
 	 */
-	id = __wt_atomic_addv64(&S2C(session)->txn_global.alloc, 1) - 1;
+	 __wt_spin_lock(session, &txn_global->id_lock);
+	id = txn_global->current;
 
 	if (publish) {
 		session->txn.id = id;
-		WT_SESSION_TXN_STATE(session)->id = id;
+		WT_PUBLISH(WT_SESSION_TXN_STATE(session)->id, id);
 	}
 
-	for (i = 0; txn_global->current != id; i++)
-		if (i < 100)
-			WT_PAUSE();
-		else
-			__wt_yield();
-
-	WT_PUBLISH(txn_global->current, id + 1);
+	++txn_global->current;
+	 __wt_spin_unlock(session, &txn_global->id_lock);
 	return (id);
 }
 
diff --git a/src/txn/txn.c b/src/txn/txn.c
index f89a8ae1b53..f9af9589172 100644
--- a/src/txn/txn.c
+++ b/src/txn/txn.c
@@ -594,8 +594,11 @@ __wt_txn_global_init(WT_SESSION_IMPL *session, const char *cfg[])
 	conn = S2C(session);
 
 	txn_global = &conn->txn_global;
-	txn_global->alloc = txn_global->current =
-	    txn_global->last_running = txn_global->oldest_id = WT_TXN_FIRST;
+	txn_global->current = txn_global->last_running =
+	    txn_global->oldest_id = WT_TXN_FIRST;
+
+	WT_RET(__wt_spin_init(session,
+	    &txn_global->id_lock, "transaction id lock"));
 
 	WT_RET(__wt_calloc_def(
 	    session, conn->session_size, &txn_global->states));
@@ -618,6 +621,6 @@ __wt_txn_global_destroy(WT_SESSION_IMPL *session)
 	conn = S2C(session);
 	txn_global = &conn->txn_global;
 
-	if (txn_global != NULL)
-		__wt_free(session, txn_global->states);
+	__wt_spin_destroy(session, &txn_global->id_lock);
+	__wt_free(session, txn_global->states);
 }