s3: Enforce a lock order in dbwrap

This makes sure we do not deadlock from doing two dbwrap_fetch_locked in two
processes in different orders. At open time, we assign a strict order to all
databases. lock_order 1 will be locked first, lock_order 2 second. No two
records of the same lock order may be locked at the same time.

4 files changed, 85 insertions, 1 deletions

diff --git a/source3/lib/dbwrap/dbwrap.c b/source3/lib/dbwrap/dbwrap.c
index c551bfdeca..e443fa5eb3 100644
--- a/source3/lib/dbwrap/dbwrap.c
+++ b/source3/lib/dbwrap/dbwrap.c
@@ -75,11 +75,90 @@ NTSTATUS dbwrap_record_delete(struct db_record *rec)
 	return rec->delete_rec(rec);
 }
 
+struct dbwrap_lock_order_state {
+	uint8_t *plock_order_mask;
+	uint8_t bitmask;
+};
+
+static int dbwrap_lock_order_state_destructor(
+	struct dbwrap_lock_order_state *s)
+{
+	*s->plock_order_mask &= ~s->bitmask;
+	return 0;
+}
+
+static struct dbwrap_lock_order_state *dbwrap_check_lock_order(
+	struct db_context *db, TALLOC_CTX *mem_ctx)
+{
+	/*
+	 * Store the lock_order of currently locked records as bits in
+	 * "lock_order_mask". We only use levels 1,2,3 right now, so a
+	 * single uint8_t is enough.
+	 */
+	static uint8_t lock_order_mask;
+
+	struct dbwrap_lock_order_state *state;
+	uint8_t idx;
+	int used;
+
+	if (db->lock_order == 0) {
+		/*
+		 * lock order 0 is for example for dbwrap_rbt without
+		 * real locking. Return state nevertheless to avoid
+		 * special cases.
+		 */
+		return talloc(mem_ctx, struct dbwrap_lock_order_state);
+	}
+
+	/*
+	 * We fill bits from the high bits, to be able to use
+	 * "ffs(lock_order_mask)"
+	 */
+	idx = sizeof(lock_order_mask)*8 - db->lock_order;
+
+	used = ffs(lock_order_mask);
+
+	DEBUG(1, ("used=%d, lock_order=%d, idx=%d\n", used,
+		  (int)db->lock_order, (int)idx));
+
+	if ((used != 0) && (used-1 <= idx)) {
+		DEBUG(0, ("Lock order violation: Trying %d, order_mask=%x\n",
+			  (int)db->lock_order, (int)lock_order_mask));
+		return NULL;
+	}
+
+	state = talloc(mem_ctx, struct dbwrap_lock_order_state);
+	if (state == NULL) {
+		DEBUG(1, ("talloc failed\n"));
+		return NULL;
+	}
+	state->bitmask = 1 << idx;
+	state->plock_order_mask = &lock_order_mask;
+
+	talloc_set_destructor(state, dbwrap_lock_order_state_destructor);
+	lock_order_mask |= state->bitmask;
+
+	return state;
+}
+
 struct db_record *dbwrap_fetch_locked(struct db_context *db,
 				      TALLOC_CTX *mem_ctx,
 				      TDB_DATA key)
 {
-	return db->fetch_locked(db, mem_ctx, key);
+	struct db_record *rec;
+	struct dbwrap_lock_order_state *lock_order;
+
+	lock_order = dbwrap_check_lock_order(db, talloc_tos());
+	if (lock_order == NULL) {
+		return NULL;
+	}
+	rec = db->fetch_locked(db, mem_ctx, key);
+	if (rec == NULL) {
+		TALLOC_FREE(lock_order);
+		return NULL;
+	}
+	(void)talloc_steal(rec, lock_order);
+	return rec;
 }
 
 struct dbwrap_fetch_state {
diff --git a/source3/lib/dbwrap/dbwrap_open.c b/source3/lib/dbwrap/dbwrap_open.c
index 23d299511b..af24ed8847 100644
--- a/source3/lib/dbwrap/dbwrap_open.c
+++ b/source3/lib/dbwrap/dbwrap_open.c
@@ -124,5 +124,8 @@ struct db_context *db_open(TALLOC_CTX *mem_ctx,
 		result = db_open_tdb(mem_ctx, name, hash_size,
 				     tdb_flags, open_flags, mode);
 	}
+	if (result != NULL) {
+		result->lock_order = lock_order;
+	}
 	return result;
 }
diff --git a/source3/lib/dbwrap/dbwrap_private.h b/source3/lib/dbwrap/dbwrap_private.h
index d0b3279368..111f02dc6b 100644
--- a/source3/lib/dbwrap/dbwrap_private.h
+++ b/source3/lib/dbwrap/dbwrap_private.h
@@ -56,6 +56,7 @@ struct db_context {
 	int (*exists)(struct db_context *db,TDB_DATA key);
 	int (*wipe)(struct db_context *db);
 	void *private_data;
+	enum dbwrap_lock_order lock_order;
 	bool persistent;
 };
 
diff --git a/source3/lib/dbwrap/dbwrap_rbt.c b/source3/lib/dbwrap/dbwrap_rbt.c
index 2460418d1d..4fbb0bc1a0 100644
--- a/source3/lib/dbwrap/dbwrap_rbt.c
+++ b/source3/lib/dbwrap/dbwrap_rbt.c
@@ -435,6 +435,7 @@ struct db_context *db_open_rbt(TALLOC_CTX *mem_ctx)
 	result->exists = db_rbt_exists;
 	result->wipe = db_rbt_wipe;
 	result->parse_record = db_rbt_parse_record;
+	result->lock_order = 0;
 
 	return result;
 }