WT-2499 Fix a race in LSM between setting and checking WT_LSM_TREE_ACTIVE.

Tree close relies on any state change being visible immediately. Found
by inspection, but could cause the symptom seen in this ticket.

1 files changed, 13 insertions, 7 deletions

diff --git a/src/lsm/lsm_tree.c b/src/lsm/lsm_tree.c
index b64b72256cc..73466a53215 100644
--- a/src/lsm/lsm_tree.c
+++ b/src/lsm/lsm_tree.c
@@ -90,8 +90,13 @@ __lsm_tree_close(WT_SESSION_IMPL *session, WT_LSM_TREE *lsm_tree, bool wait)
 	WT_DECL_RET;
 	int i;
 
-	/* Stop any active merges. */
-	F_CLR(lsm_tree, WT_LSM_TREE_ACTIVE);
+	/*
+	 * Stop any new work units being added. The barrier is necessary
+	 * because we rely on the state change being visible before checking
+	 * the tree queue state.
+	 */
+	lsm_tree->active = false;
+	WT_READ_BARRIER();
 
 	/*
 	 * Wait for all LSM operations to drain. If WiredTiger is shutting
@@ -123,7 +128,7 @@ __lsm_tree_close(WT_SESSION_IMPL *session, WT_LSM_TREE *lsm_tree, bool wait)
 	}
 	return (0);
 
-err:	F_SET(lsm_tree, WT_LSM_TREE_ACTIVE);
+err:	lsm_tree->active = true;
 	return (ret);
 }
 
@@ -389,7 +394,7 @@ __lsm_tree_find(WT_SESSION_IMPL *session,
 				    lsm_tree->refcnt != 1) {
 					(void)__wt_atomic_sub32(
 					    &lsm_tree->refcnt, 1);
-					F_SET(lsm_tree, WT_LSM_TREE_ACTIVE);
+					lsm_tree->active = true;
 					lsm_tree->excl_session = NULL;
 					return (EBUSY);
 				}
@@ -504,7 +509,8 @@ __lsm_tree_open(WT_SESSION_IMPL *session,
 
 	/* Now the tree is setup, make it visible to others. */
 	TAILQ_INSERT_HEAD(&S2C(session)->lsmqh, lsm_tree, q);
-	F_SET(lsm_tree, WT_LSM_TREE_ACTIVE | WT_LSM_TREE_OPEN);
+	lsm_tree->active = true;
+	F_SET(lsm_tree, WT_LSM_TREE_OPEN);
 
 	*treep = lsm_tree;
 
@@ -545,7 +551,7 @@ __wt_lsm_tree_release(WT_SESSION_IMPL *session, WT_LSM_TREE *lsm_tree)
 	WT_ASSERT(session, lsm_tree->refcnt > 0);
 	if (lsm_tree->excl_session == session) {
 		/* We cleared the active flag when getting exclusive access. */
-		F_SET(lsm_tree, WT_LSM_TREE_ACTIVE);
+		lsm_tree->active = true;
 		lsm_tree->excl_session = NULL;
 	}
 	(void)__wt_atomic_sub32(&lsm_tree->refcnt, 1);
@@ -1218,7 +1224,7 @@ __wt_lsm_compact(WT_SESSION_IMPL *session, const char *name, bool *skipp)
 	}
 
 	/* Wait for the work unit queues to drain. */
-	while (F_ISSET(lsm_tree, WT_LSM_TREE_ACTIVE)) {
+	while (lsm_tree->active) {
 		/*
 		 * The flush flag is cleared when the chunk has been flushed.
 		 * Continue to push forced flushes until the chunk is on disk.