ITS#7969 LMDB: volatile & __synchronize().

Globally shared fields of meta-data are not 'volatile'.

libraries/liblmdb/mdb.c

@@ -610,11 +610,11 @@ typedef struct MDB_rxbody {
 	 *	started from so we can avoid overwriting any data used in that
 	 *	particular version.
 	 */
-	txnid_t		mrb_txnid;
+	volatile txnid_t		mrb_txnid;
 	/** The process ID of the process owning this reader txn. */
-	MDB_PID_T	mrb_pid;
+	volatile MDB_PID_T	mrb_pid;
 	/** The thread ID of the thread owning this txn. */
-	MDB_THR_T	mrb_tid;
+	volatile MDB_THR_T	mrb_tid;
 } MDB_rxbody;
 
 	/** The actual reader record, with cacheline padding. */
@@ -664,12 +664,12 @@ typedef struct MDB_txbody {
 		 *	This is recorded here only for convenience; the value can always
 		 *	be determined by reading the main database meta pages.
 		 */
-	txnid_t		mtb_txnid;
+	volatile txnid_t		mtb_txnid;
 		/** The number of slots that have been used in the reader table.
 		 *	This always records the maximum count, it is not decremented
 		 *	when readers release their slots.
 		 */
-	unsigned	mtb_numreaders;
+	volatile unsigned	mtb_numreaders;
 } MDB_txbody;
 
 	/** The actual reader table definition. */
@@ -942,7 +942,7 @@ typedef struct MDB_meta {
 	/** Any persistent environment flags. @ref mdb_env */
 #define	mm_flags	mm_dbs[0].md_flags
 	pgno_t		mm_last_pg;			/**< last used page in file */
-	txnid_t		mm_txnid;			/**< txnid that committed this page */
+	volatile txnid_t	mm_txnid;	/**< txnid that committed this page */
 } MDB_meta;
 
 	/** Buffer for a stack-allocated meta page.
@@ -3601,6 +3601,10 @@ mdb_env_write_meta(MDB_txn *txn)
 		mp->mm_dbs[0] = txn->mt_dbs[0];
 		mp->mm_dbs[1] = txn->mt_dbs[1];
 		mp->mm_last_pg = txn->mt_next_pgno - 1;
+#if !(defined(_MSC_VER) || defined(__i386__) || defined(__x86_64__))
+		/* LY: issue a memory barrier, if not x86. ITS#7969 */
+		__sync_synchronize();
+#endif
 		mp->mm_txnid = txn->mt_txnid;
 		if (!(env->me_flags & (MDB_NOMETASYNC|MDB_NOSYNC))) {
 			unsigned meta_size = env->me_psize;