1 files changed, 24 insertions, 4 deletions

diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index 8bda474bc71..09b7311e7bc 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -654,17 +654,28 @@ ProcArrayApplyRecoveryInfo(RunningTransactions running)
 							  running->latestCompletedXid))
 		ShmemVariableCache->latestCompletedXid = running->latestCompletedXid;
 
-	/* nextXid must be beyond any observed xid */
+	Assert(TransactionIdIsNormal(ShmemVariableCache->latestCompletedXid));
+
+	LWLockRelease(ProcArrayLock);
+
+	/*
+	 * ShmemVariableCache->nextXid must be beyond any observed xid.
+	 *
+	 * We don't expect anyone else to modify nextXid, hence we don't need to
+	 * hold a lock while examining it.  We still acquire the lock to modify
+	 * it, though.
+	 */
 	nextXid = latestObservedXid;
 	TransactionIdAdvance(nextXid);
 	if (TransactionIdFollows(nextXid, ShmemVariableCache->nextXid))
+	{
+		LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
 		ShmemVariableCache->nextXid = nextXid;
+		LWLockRelease(XidGenLock);
+	}
 
-	Assert(TransactionIdIsNormal(ShmemVariableCache->latestCompletedXid));
 	Assert(TransactionIdIsValid(ShmemVariableCache->nextXid));
 
-	LWLockRelease(ProcArrayLock);
-
 	KnownAssignedXidsDisplay(trace_recovery(DEBUG3));
 	if (standbyState == STANDBY_SNAPSHOT_READY)
 		elog(trace_recovery(DEBUG1), "recovery snapshots are now enabled");
@@ -1690,6 +1701,13 @@ GetOldestActiveTransactionId(void)
 
 	LWLockAcquire(ProcArrayLock, LW_SHARED);
 
+	/*
+	 * It's okay to read nextXid without acquiring XidGenLock because (1) we
+	 * assume TransactionIds can be read atomically and (2) we don't care if
+	 * we get a slightly stale value.  It can't be very stale anyway, because
+	 * the LWLockAcquire above will have done any necessary memory
+	 * interlocking.
+	 */
 	oldestRunningXid = ShmemVariableCache->nextXid;
 
 	/*
@@ -2609,7 +2627,9 @@ RecordKnownAssignedTransactionIds(TransactionId xid)
 		/* ShmemVariableCache->nextXid must be beyond any observed xid */
 		next_expected_xid = latestObservedXid;
 		TransactionIdAdvance(next_expected_xid);
+		LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
 		ShmemVariableCache->nextXid = next_expected_xid;
+		LWLockRelease(XidGenLock);
 	}
 }