Fix locking while setting flags in MySerializableXact.

Even if a flag is modified only by the backend owning the transaction, it's
not safe to modify it without a lock. Another backend might be setting or
clearing a different flag in the flags field concurrently, and that
operation might be lost because setting or clearing a bit in a word is not
atomic.

Make did-write flag a simple backend-private boolean variable, because it
was only set or tested in the owning backend (except when committing a
prepared transaction, but it's not worthwhile to optimize for the case of a
read-only prepared transaction). This also eliminates the need to add
locking where that flag is set.

Also, set the did-write flag when doing DDL operations like DROP TABLE or
TRUNCATE -- that was missed earlier.

diff --git a/src/backend/storage/lmgr/predicate.c b/src/backend/storage/lmgr/predicate.c
index d5cb7da1b0d41f27729612fd0f1e87e8aab0fba8..9f79f54bf7c9f87a0721f36d0d6e0294af2e0f68 100644 (file)
--- a/src/backend/storage/lmgr/predicate.c
+++ b/src/backend/storage/lmgr/predicate.c
@@ -392,10 +392,11 @@ static HTAB *LocalPredicateLockHash = NULL;
 
 /*
  * Keep a pointer to the currently-running serializable transaction (if any)
- * for quick reference.
- * TODO SSI: Remove volatile qualifier and the then-unnecessary casts?
+ * for quick reference. Also, remember if we have written anything that could
+ * cause a rw-conflict.
  */
-static volatile SERIALIZABLEXACT *MySerializableXact = InvalidSerializableXact;
+static SERIALIZABLEXACT *MySerializableXact = InvalidSerializableXact;
+static bool MyXactDidWrite = false;
 
 /* local functions */
 
@@ -1424,20 +1425,30 @@ GetSafeSnapshot(Snapshot origSnapshot)
        if (MySerializableXact == InvalidSerializableXact)
            return snapshot;    /* no concurrent r/w xacts; it's safe */
 
-       MySerializableXact->flags |= SXACT_FLAG_DEFERRABLE_WAITING;
+       LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
 
        /*
         * Wait for concurrent transactions to finish. Stop early if one of
         * them marked us as conflicted.
         */
+       MySerializableXact->flags |= SXACT_FLAG_DEFERRABLE_WAITING;
        while (!(SHMQueueEmpty((SHM_QUEUE *)
                             &MySerializableXact->possibleUnsafeConflicts) ||
                 SxactIsROUnsafe(MySerializableXact)))
+       {
+           LWLockRelease(SerializableXactHashLock);
            ProcWaitForSignal();
-
+           LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
+       }
        MySerializableXact->flags &= ~SXACT_FLAG_DEFERRABLE_WAITING;
+
        if (!SxactIsROUnsafe(MySerializableXact))
+       {
+           LWLockRelease(SerializableXactHashLock);
            break;              /* success */
+       }
+
+       LWLockRelease(SerializableXactHashLock);
 
        /* else, need to retry... */
        ereport(DEBUG2,
@@ -1600,6 +1611,7 @@ RegisterSerializableTransactionInt(Snapshot snapshot)
    }
 
    MySerializableXact = sxact;
+   MyXactDidWrite = false;     /* haven't written anything yet */
 
    LWLockRelease(SerializableXactHashLock);
 
@@ -1635,24 +1647,24 @@ RegisterPredicateLockingXid(const TransactionId xid)
    if (MySerializableXact == InvalidSerializableXact)
        return;
 
-   /* This should only be done once per transaction. */
-   Assert(MySerializableXact->topXid == InvalidTransactionId);
-
    /* We should have a valid XID and be at the top level. */
    Assert(TransactionIdIsValid(xid));
 
+   LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
+
+   /* This should only be done once per transaction. */
+   Assert(MySerializableXact->topXid == InvalidTransactionId);
+
    MySerializableXact->topXid = xid;
 
    sxidtag.xid = xid;
-   LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
    sxid = (SERIALIZABLEXID *) hash_search(SerializableXidHash,
                                           &sxidtag,
                                           HASH_ENTER, &found);
-   Assert(sxid != NULL);
    Assert(!found);
 
    /* Initialize the structure. */
-   sxid->myXact = (SERIALIZABLEXACT *) MySerializableXact;
+   sxid->myXact = MySerializableXact;
    LWLockRelease(SerializableXactHashLock);
 }
 
@@ -1881,7 +1893,7 @@ DeleteChildTargetLocks(const PREDICATELOCKTARGETTAG *newtargettag)
    PREDICATELOCK *predlock;
 
    LWLockAcquire(SerializablePredicateLockListLock, LW_SHARED);
-   sxact = (SERIALIZABLEXACT *) MySerializableXact;
+   sxact = MySerializableXact;
    predlock = (PREDICATELOCK *)
        SHMQueueNext(&(sxact->predicateLocks),
                     &(sxact->predicateLocks),
@@ -2200,8 +2212,7 @@ PredicateLockAcquire(const PREDICATELOCKTARGETTAG *targettag)
        locallock->childLocks = 0;
 
    /* Actually create the lock */
-   CreatePredicateLock(targettag, targettaghash,
-                       (SERIALIZABLEXACT *) MySerializableXact);
+   CreatePredicateLock(targettag, targettaghash, MySerializableXact);
 
    /*
     * Lock has been acquired. Check whether it should be promoted to a
@@ -3042,7 +3053,7 @@ ReleasePredicateLocks(const bool isCommit)
        Assert(IsolationIsSerializable());
 
    /* We'd better not already be on the cleanup list. */
-   Assert(!SxactIsOnFinishedList((SERIALIZABLEXACT *) MySerializableXact));
+   Assert(!SxactIsOnFinishedList(MySerializableXact));
 
    topLevelIsDeclaredReadOnly = SxactIsReadOnly(MySerializableXact);
 
@@ -3070,7 +3081,7 @@ ReleasePredicateLocks(const bool isCommit)
        MySerializableXact->flags |= SXACT_FLAG_COMMITTED;
        MySerializableXact->commitSeqNo = ++(PredXact->LastSxactCommitSeqNo);
        /* Recognize implicit read-only transaction (commit without write). */
-       if (!(MySerializableXact->flags & SXACT_FLAG_DID_WRITE))
+       if (!MyXactDidWrite)
            MySerializableXact->flags |= SXACT_FLAG_READ_ONLY;
    }
    else
@@ -3218,7 +3229,7 @@ ReleasePredicateLocks(const bool isCommit)
 
            /* Mark conflicted if necessary. */
            if (isCommit
-               && (MySerializableXact->flags & SXACT_FLAG_DID_WRITE)
+               && MyXactDidWrite
                && SxactHasConflictOut(MySerializableXact)
                && (MySerializableXact->SeqNo.earliestOutConflictCommit
                    <= roXact->SeqNo.lastCommitBeforeSnapshot))
@@ -3282,8 +3293,7 @@ ReleasePredicateLocks(const bool isCommit)
                          (SHM_QUEUE *) &(MySerializableXact->finishedLink));
 
    if (!isCommit)
-       ReleaseOneSerializableXact((SERIALIZABLEXACT *) MySerializableXact,
-                                  false, false);
+       ReleaseOneSerializableXact(MySerializableXact, false, false);
 
    LWLockRelease(SerializableFinishedListLock);
 
@@ -3291,6 +3301,7 @@ ReleasePredicateLocks(const bool isCommit)
        ClearOldPredicateLocks();
 
    MySerializableXact = InvalidSerializableXact;
+   MyXactDidWrite = false;
 
    /* Delete per-transaction lock table */
    if (LocalPredicateLockHash != NULL)
@@ -3851,7 +3862,7 @@ CheckForSerializableConflictOut(const bool visible, const Relation relation,
        return;
    }
 
-   if (RWConflictExists((SERIALIZABLEXACT *) MySerializableXact, sxact))
+   if (RWConflictExists(MySerializableXact, sxact))
    {
        /* We don't want duplicate conflict records in the list. */
        LWLockRelease(SerializableXactHashLock);
@@ -3862,7 +3873,7 @@ CheckForSerializableConflictOut(const bool visible, const Relation relation,
     * Flag the conflict.  But first, if this conflict creates a dangerous
     * structure, ereport an error.
     */
-   FlagRWConflict((SERIALIZABLEXACT *) MySerializableXact, sxact);
+   FlagRWConflict(MySerializableXact, sxact);
    LWLockRelease(SerializableXactHashLock);
 }
 
@@ -3944,7 +3955,7 @@ CheckTargetForConflictsIn(PREDICATELOCKTARGETTAG *targettag)
                 && (!SxactIsCommitted(sxact)
                     || TransactionIdPrecedes(GetTransactionSnapshot()->xmin,
                                              sxact->finishedBefore))
-       && !RWConflictExists(sxact, (SERIALIZABLEXACT *) MySerializableXact))
+                && !RWConflictExists(sxact, MySerializableXact))
        {
            LWLockRelease(SerializableXactHashLock);
            LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
@@ -3957,10 +3968,9 @@ CheckTargetForConflictsIn(PREDICATELOCKTARGETTAG *targettag)
                && (!SxactIsCommitted(sxact)
                    || TransactionIdPrecedes(GetTransactionSnapshot()->xmin,
                                             sxact->finishedBefore))
-               && !RWConflictExists(sxact,
-                                    (SERIALIZABLEXACT *) MySerializableXact))
+               && !RWConflictExists(sxact, MySerializableXact))
            {
-               FlagRWConflict(sxact, (SERIALIZABLEXACT *) MySerializableXact);
+               FlagRWConflict(sxact, MySerializableXact);
            }
 
            LWLockRelease(SerializableXactHashLock);
@@ -4065,7 +4075,11 @@ CheckForSerializableConflictIn(const Relation relation, const HeapTuple tuple,
                 errdetail("Cancelled on identification as a pivot, during conflict in checking."),
                 errhint("The transaction might succeed if retried.")));
 
-   MySerializableXact->flags |= SXACT_FLAG_DID_WRITE;
+   /*
+    * We're doing a write which might cause rw-conflicts now or later.
+    * Memorize that fact.
+    */
+   MyXactDidWrite = true;
 
    /*
     * It is important that we check for locks from the finest granularity to
@@ -4150,6 +4164,12 @@ CheckTableForSerializableConflictIn(const Relation relation)
    if (SkipSerialization(relation))
        return;
 
+   /*
+    * We're doing a write which might cause rw-conflicts now or later.
+    * Memorize that fact.
+    */
+   MyXactDidWrite = true;
+
    Assert(relation->rd_index == NULL); /* not an index relation */
 
    dbId = relation->rd_node.dbNode;
@@ -4192,10 +4212,10 @@ CheckTableForSerializableConflictIn(const Relation relation)
                             offsetof(PREDICATELOCK, targetLink));
 
            if (predlock->tag.myXact != MySerializableXact
-               && !RWConflictExists(predlock->tag.myXact,
-                                    (SERIALIZABLEXACT *) MySerializableXact))
-               FlagRWConflict(predlock->tag.myXact,
-                              (SERIALIZABLEXACT *) MySerializableXact);
+             && !RWConflictExists(predlock->tag.myXact, MySerializableXact))
+           {
+               FlagRWConflict(predlock->tag.myXact, MySerializableXact);
+           }
 
            predlock = nextpredlock;
        }
@@ -4506,7 +4526,7 @@ AtPrepare_PredicateLocks(void)
    TwoPhasePredicateXactRecord *xactRecord;
    TwoPhasePredicateLockRecord *lockRecord;
 
-   sxact = (SERIALIZABLEXACT *) MySerializableXact;
+   sxact = MySerializableXact;
    xactRecord = &(record.data.xactRecord);
    lockRecord = &(record.data.lockRecord);
 
@@ -4583,6 +4603,7 @@ PostPrepare_PredicateLocks(TransactionId xid)
    LocalPredicateLockHash = NULL;
 
    MySerializableXact = InvalidSerializableXact;
+   MyXactDidWrite = false;
 }
 
 /*
@@ -4609,6 +4630,8 @@ PredicateLockTwoPhaseFinish(TransactionId xid, bool isCommit)
 
    /* Release its locks */
    MySerializableXact = sxid->myXact;
+   MyXactDidWrite = true;      /* conservatively assume that we wrote
+                                * something */
    ReleasePredicateLocks(isCommit);
 }
 

diff --git a/src/include/storage/predicate_internals.h b/src/include/storage/predicate_internals.h
index e73159554b380e0becf7265b06febf26ff83a013..da6e641f4e4ec0f18e11e7f1024c1a37cdd345a7 100644 (file)
--- a/src/include/storage/predicate_internals.h
+++ b/src/include/storage/predicate_internals.h
@@ -99,14 +99,13 @@ typedef struct SERIALIZABLEXACT
  */
 #define SXACT_FLAG_CONFLICT_OUT                0x00000004
 #define SXACT_FLAG_READ_ONLY               0x00000008
-#define SXACT_FLAG_DID_WRITE               0x00000010
-#define SXACT_FLAG_MARKED_FOR_DEATH            0x00000020
-#define SXACT_FLAG_DEFERRABLE_WAITING      0x00000040
-#define SXACT_FLAG_RO_SAFE                 0x00000080
-#define SXACT_FLAG_RO_UNSAFE               0x00000100
-#define SXACT_FLAG_SUMMARY_CONFLICT_IN     0x00000200
-#define SXACT_FLAG_SUMMARY_CONFLICT_OUT        0x00000400
-#define SXACT_FLAG_PREPARED                    0x00000800
+#define SXACT_FLAG_MARKED_FOR_DEATH            0x00000010
+#define SXACT_FLAG_DEFERRABLE_WAITING      0x00000020
+#define SXACT_FLAG_RO_SAFE                 0x00000040
+#define SXACT_FLAG_RO_UNSAFE               0x00000080
+#define SXACT_FLAG_SUMMARY_CONFLICT_IN     0x00000100
+#define SXACT_FLAG_SUMMARY_CONFLICT_OUT        0x00000200
+#define SXACT_FLAG_PREPARED                    0x00000400
 
 /*
  * The following types are used to provide an ad hoc list for holding