Rename PGPROC fields related to group XID clearing again.

Commit 0e141c0fbb introduced a new
facility to reduce ProcArrayLock contention by clearing several XIDs
from the ProcArray under a single lock acquisition.  The names
initially chosen were deemed not to be very good choices, so commit
4aec49899e renamed them.  But now it
seems like we still didn't get it right.  A pending patch wants to
add similar infrastructure for batching CLOG updates, so the names
need to be clear enough to allow a new set of structure members with
a related purpose.

Amit Kapila

src/backend/storage/ipc/procarray.c

@@ -497,14 +497,14 @@ ProcArrayGroupClearXid(PGPROC *proc, TransactionId latestXid)
 	Assert(TransactionIdIsValid(allPgXact[proc->pgprocno].xid));
 
 	/* Add ourselves to the list of processes needing a group XID clear. */
-	proc->clearXid = true;
-	proc->backendLatestXid = latestXid;
+	proc->procArrayGroupMember = true;
+	proc->procArrayGroupMemberXid = latestXid;
 	while (true)
 	{
-		nextidx = pg_atomic_read_u32(&procglobal->firstClearXidElem);
-		pg_atomic_write_u32(&proc->nextClearXidElem, nextidx);
+		nextidx = pg_atomic_read_u32(&procglobal->procArrayGroupFirst);
+		pg_atomic_write_u32(&proc->procArrayGroupNext, nextidx);
 
-		if (pg_atomic_compare_exchange_u32(&procglobal->firstClearXidElem,
+		if (pg_atomic_compare_exchange_u32(&procglobal->procArrayGroupFirst,
 										   &nextidx,
 										   (uint32) proc->pgprocno))
 			break;
@@ -523,12 +523,12 @@ ProcArrayGroupClearXid(PGPROC *proc, TransactionId latestXid)
 		{
 			/* acts as a read barrier */
 			PGSemaphoreLock(&proc->sem);
-			if (!proc->clearXid)
+			if (!proc->procArrayGroupMember)
 				break;
 			extraWaits++;
 		}
 
-		Assert(pg_atomic_read_u32(&proc->nextClearXidElem) == INVALID_PGPROCNO);
+		Assert(pg_atomic_read_u32(&proc->procArrayGroupNext) == INVALID_PGPROCNO);
 
 		/* Fix semaphore count for any absorbed wakeups */
 		while (extraWaits-- > 0)
@@ -546,8 +546,8 @@ ProcArrayGroupClearXid(PGPROC *proc, TransactionId latestXid)
 	 */
 	while (true)
 	{
-		nextidx = pg_atomic_read_u32(&procglobal->firstClearXidElem);
-		if (pg_atomic_compare_exchange_u32(&procglobal->firstClearXidElem,
+		nextidx = pg_atomic_read_u32(&procglobal->procArrayGroupFirst);
+		if (pg_atomic_compare_exchange_u32(&procglobal->procArrayGroupFirst,
 										   &nextidx,
 										   INVALID_PGPROCNO))
 			break;
@@ -562,10 +562,10 @@ ProcArrayGroupClearXid(PGPROC *proc, TransactionId latestXid)
 		PGPROC	*proc = &allProcs[nextidx];
 		PGXACT	*pgxact = &allPgXact[nextidx];
 
-		ProcArrayEndTransactionInternal(proc, pgxact, proc->backendLatestXid);
+		ProcArrayEndTransactionInternal(proc, pgxact, proc->procArrayGroupMemberXid);
 
 		/* Move to next proc in list. */
-		nextidx = pg_atomic_read_u32(&proc->nextClearXidElem);
+		nextidx = pg_atomic_read_u32(&proc->procArrayGroupNext);
 	}
 
 	/* We're done with the lock now. */
@@ -582,13 +582,13 @@ ProcArrayGroupClearXid(PGPROC *proc, TransactionId latestXid)
 	{
 		PGPROC	*proc = &allProcs[wakeidx];
 
-		wakeidx = pg_atomic_read_u32(&proc->nextClearXidElem);
-		pg_atomic_write_u32(&proc->nextClearXidElem, INVALID_PGPROCNO);
+		wakeidx = pg_atomic_read_u32(&proc->procArrayGroupNext);
+		pg_atomic_write_u32(&proc->procArrayGroupNext, INVALID_PGPROCNO);
 
 		/* ensure all previous writes are visible before follower continues. */
 		pg_write_barrier();
 
-		proc->clearXid = false;
+		proc->procArrayGroupMember = false;
 
 		if (proc != MyProc)
 			PGSemaphoreUnlock(&proc->sem);

src/backend/storage/lmgr/proc.c

@@ -181,7 +181,7 @@ InitProcGlobal(void)
 	ProcGlobal->startupBufferPinWaitBufId = -1;
 	ProcGlobal->walwriterLatch = NULL;
 	ProcGlobal->checkpointerLatch = NULL;
-	pg_atomic_init_u32(&ProcGlobal->firstClearXidElem, INVALID_PGPROCNO);
+	pg_atomic_init_u32(&ProcGlobal->procArrayGroupFirst, INVALID_PGPROCNO);
 
 	/*
 	 * Create and initialize all the PGPROC structures we'll need.  There are
@@ -396,9 +396,9 @@ InitProcess(void)
 	SHMQueueElemInit(&(MyProc->syncRepLinks));
 
 	/* Initialize fields for group XID clearing. */
-	MyProc->clearXid = false;
-	MyProc->backendLatestXid = InvalidTransactionId;
-	pg_atomic_init_u32(&MyProc->nextClearXidElem, INVALID_PGPROCNO);
+	MyProc->procArrayGroupMember = false;
+	MyProc->procArrayGroupMemberXid = InvalidTransactionId;
+	pg_atomic_init_u32(&MyProc->procArrayGroupNext, INVALID_PGPROCNO);
 
 	/* Check that group locking fields are in a proper initial state. */
 	Assert(MyProc->lockGroupLeaderIdentifier == 0);

src/include/storage/proc.h

@@ -142,9 +142,15 @@ struct PGPROC
 	struct XidCache subxids;	/* cache for subtransaction XIDs */
 
 	/* Support for group XID clearing. */
-	bool			clearXid;
-	pg_atomic_uint32	nextClearXidElem;
-	TransactionId	backendLatestXid;
+	/* true, if member of ProcArray group waiting for XID clear */
+	bool			procArrayGroupMember;
+	/* next ProcArray group member waiting for XID clear */
+	pg_atomic_uint32	procArrayGroupNext;
+	/*
+	 * latest transaction id among the transaction's main XID and
+	 * subtransactions
+	 */
+	TransactionId	procArrayGroupMemberXid;
 
 	/* Per-backend LWLock.  Protects fields below. */
 	LWLock		backendLock;
@@ -217,7 +223,7 @@ typedef struct PROC_HDR
 	/* Head of list of bgworker free PGPROC structures */
 	PGPROC	   *bgworkerFreeProcs;
 	/* First pgproc waiting for group XID clear */
-	pg_atomic_uint32 firstClearXidElem;
+	pg_atomic_uint32 procArrayGroupFirst;
 	/* WALWriter process's latch */
 	Latch	   *walwriterLatch;
 	/* Checkpointer process's latch */