From 4a0b30499caab8628efcedb8be5643fead7cd17e Mon Sep 17 00:00:00 2001
From: Hallvard Furuseth <hallvard@openldap.org>
Date: Thu, 12 Apr 2012 07:34:13 +0200
Subject: [PATCH] libmdb: Do not use the MDB_txbody.mtb_txnid field.
Get txn->mt_txnid from the meta page instead. Otherwise it
and txn->mt_toggle could become inconsistent, since read-only
txns fetched both from MDB_txbody without mutex proteciton.
---
libraries/libmdb/mdb.c | 16 ++++++++++++----
1 file changed, 12 insertions(+), 4 deletions(-)
diff --git a/libraries/libmdb/mdb.c b/libraries/libmdb/mdb.c
index ba6ded3226..8280e22afb 100644
--- a/libraries/libmdb/mdb.c
+++ b/libraries/libmdb/mdb.c
@@ -511,11 +511,17 @@ typedef struct MDB_txbody {
*/
pthread_mutex_t mtb_mutex;
#endif
+#if MDB_VERSION == 1
/** The ID of the last transaction committed to the database.
* This is recorded here only for convenience; the value can always
* be determined by reading the main database meta pages.
+ *
+ * Value is unused, but maintained for backwards compatibility.
+ * Drop this or mtb_me_toggle when changing MDB_VERSION.
+ * (Reading both should have been done atomically.)
*/
txnid_t mtb_txnid;
+#endif
/** 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.
@@ -536,7 +542,9 @@ typedef struct MDB_txninfo {
#define mti_version mt1.mtb.mtb_version
#define mti_mutex mt1.mtb.mtb_mutex
#define mti_rmname mt1.mtb.mtb_rmname
+#if MDB_VERSION == 1
#define mti_txnid mt1.mtb.mtb_txnid
+#endif
#define mti_numreaders mt1.mtb.mtb_numreaders
#define mti_me_toggle mt1.mtb.mtb_me_toggle
char pad[(sizeof(MDB_txbody)+CACHELINE-1) & ~(CACHELINE-1)];
@@ -1565,18 +1573,19 @@ mdb_txn_renew0(MDB_txn *txn)
pthread_setspecific(env->me_txkey, r);
}
txn->mt_toggle = env->me_txns->mti_me_toggle;
- txn->mt_txnid = env->me_txns->mti_txnid;
+ txn->mt_txnid = r->mr_txnid = env->me_metas[txn->mt_toggle]->mm_txnid;
+
/* This happens if a different process was the
* last writer to the DB.
*/
if (env->me_wtxnid < txn->mt_txnid)
mt_dbflag = DB_STALE;
- r->mr_txnid = txn->mt_txnid;
txn->mt_u.reader = r;
} else {
LOCK_MUTEX_W(env);
- txn->mt_txnid = env->me_txns->mti_txnid;
+ txn->mt_toggle = env->me_txns->mti_me_toggle;
+ txn->mt_txnid = env->me_metas[txn->mt_toggle]->mm_txnid;
if (env->me_wtxnid < txn->mt_txnid)
mt_dbflag = DB_STALE;
txn->mt_txnid++;
@@ -1584,7 +1593,6 @@ mdb_txn_renew0(MDB_txn *txn)
if (txn->mt_txnid == mdb_debug_start)
mdb_debug = 1;
#endif
- txn->mt_toggle = env->me_txns->mti_me_toggle;
txn->mt_u.dirty_list = env->me_dirty_list;
txn->mt_u.dirty_list[0].mid = 0;
txn->mt_free_pgs = env->me_free_pgs;
--
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