Use 64-bit atomics for xlblocks array elements.

In preparation for reading the contents of WAL buffers without a
lock. Also, avoids the previously-needed comment in GetXLogBuffer()
explaining why it's safe from torn reads.

Author: Bharath Rupireddy
Discussion: https://postgr.es/m/CALj2ACVfFMfqD5oLzZSQQZWfXiJqd-NdX0_317veP6FuB31QWA@mail.gmail.com
Reviewed-by: Andres Freund

Author: jeff-davis

src/backend/access/transam/xlog.c

@@ -501,7 +501,7 @@ typedef struct XLogCtlData
 	 * WALBufMappingLock.
 	 */
 	char	   *pages;			/* buffers for unwritten XLOG pages */
-	XLogRecPtr *xlblocks;		/* 1st byte ptr-s + XLOG_BLCKSZ */
+	pg_atomic_uint64 *xlblocks; /* 1st byte ptr-s + XLOG_BLCKSZ */
 	int			XLogCacheBlck;	/* highest allocated xlog buffer index */
 
 	/*
@@ -1636,20 +1636,16 @@ GetXLogBuffer(XLogRecPtr ptr, TimeLineID tli)
 	 * out to disk and evicted, and the caller is responsible for making sure
 	 * that doesn't happen.
 	 *
-	 * However, we don't hold a lock while we read the value. If someone has
-	 * just initialized the page, it's possible that we get a "torn read" of
-	 * the XLogRecPtr if 64-bit fetches are not atomic on this platform. In
-	 * that case we will see a bogus value. That's ok, we'll grab the mapping
-	 * lock (in AdvanceXLInsertBuffer) and retry if we see anything else than
-	 * the page we're looking for. But it means that when we do this unlocked
-	 * read, we might see a value that appears to be ahead of the page we're
-	 * looking for. Don't PANIC on that, until we've verified the value while
-	 * holding the lock.
+	 * We don't hold a lock while we read the value. If someone is just about
+	 * to initialize or has just initialized the page, it's possible that we
+	 * get InvalidXLogRecPtr. That's ok, we'll grab the mapping lock (in
+	 * AdvanceXLInsertBuffer) and retry if we see anything other than the page
+	 * we're looking for.
 	 */
 	expectedEndPtr = ptr;
 	expectedEndPtr += XLOG_BLCKSZ - ptr % XLOG_BLCKSZ;
 
-	endptr = XLogCtl->xlblocks[idx];
+	endptr = pg_atomic_read_u64(&XLogCtl->xlblocks[idx]);
 	if (expectedEndPtr != endptr)
 	{
 		XLogRecPtr	initializedUpto;
@@ -1680,7 +1676,7 @@ GetXLogBuffer(XLogRecPtr ptr, TimeLineID tli)
 		WALInsertLockUpdateInsertingAt(initializedUpto);
 
 		AdvanceXLInsertBuffer(ptr, tli, false);
-		endptr = XLogCtl->xlblocks[idx];
+		endptr = pg_atomic_read_u64(&XLogCtl->xlblocks[idx]);
 
 		if (expectedEndPtr != endptr)
 			elog(PANIC, "could not find WAL buffer for %X/%X",
@@ -1867,7 +1863,7 @@ AdvanceXLInsertBuffer(XLogRecPtr upto, TimeLineID tli, bool opportunistic)
 		 * be zero if the buffer hasn't been used yet).  Fall through if it's
 		 * already written out.
 		 */
-		OldPageRqstPtr = XLogCtl->xlblocks[nextidx];
+		OldPageRqstPtr = pg_atomic_read_u64(&XLogCtl->xlblocks[nextidx]);
 		if (LogwrtResult.Write < OldPageRqstPtr)
 		{
 			/*
@@ -1989,8 +1985,7 @@ AdvanceXLInsertBuffer(XLogRecPtr upto, TimeLineID tli, bool opportunistic)
 		 */
 		pg_write_barrier();
 
-		*((volatile XLogRecPtr *) &XLogCtl->xlblocks[nextidx]) = NewPageEndPtr;
-
+		pg_atomic_write_u64(&XLogCtl->xlblocks[nextidx], NewPageEndPtr);
 		XLogCtl->InitializedUpTo = NewPageEndPtr;
 
 		npages++;
@@ -2208,7 +2203,7 @@ XLogWrite(XLogwrtRqst WriteRqst, TimeLineID tli, bool flexible)
 		 * if we're passed a bogus WriteRqst.Write that is past the end of the
 		 * last page that's been initialized by AdvanceXLInsertBuffer.
 		 */
-		XLogRecPtr	EndPtr = XLogCtl->xlblocks[curridx];
+		XLogRecPtr	EndPtr = pg_atomic_read_u64(&XLogCtl->xlblocks[curridx]);
 
 		if (LogwrtResult.Write >= EndPtr)
 			elog(PANIC, "xlog write request %X/%X is past end of log %X/%X",
@@ -4632,7 +4627,7 @@ XLOGShmemSize(void)
 	/* WAL insertion locks, plus alignment */
 	size = add_size(size, mul_size(sizeof(WALInsertLockPadded), NUM_XLOGINSERT_LOCKS + 1));
 	/* xlblocks array */
-	size = add_size(size, mul_size(sizeof(XLogRecPtr), XLOGbuffers));
+	size = add_size(size, mul_size(sizeof(pg_atomic_uint64), XLOGbuffers));
 	/* extra alignment padding for XLOG I/O buffers */
 	size = add_size(size, Max(XLOG_BLCKSZ, PG_IO_ALIGN_SIZE));
 	/* and the buffers themselves */
@@ -4710,10 +4705,13 @@ XLOGShmemInit(void)
 	 * needed here.
 	 */
 	allocptr = ((char *) XLogCtl) + sizeof(XLogCtlData);
-	XLogCtl->xlblocks = (XLogRecPtr *) allocptr;
-	memset(XLogCtl->xlblocks, 0, sizeof(XLogRecPtr) * XLOGbuffers);
-	allocptr += sizeof(XLogRecPtr) * XLOGbuffers;
+	XLogCtl->xlblocks = (pg_atomic_uint64 *) allocptr;
+	allocptr += sizeof(pg_atomic_uint64) * XLOGbuffers;
 
+	for (i = 0; i < XLOGbuffers; i++)
+	{
+		pg_atomic_init_u64(&XLogCtl->xlblocks[i], InvalidXLogRecPtr);
+	}
 
 	/* WAL insertion locks. Ensure they're aligned to the full padded size */
 	allocptr += sizeof(WALInsertLockPadded) -
@@ -5750,7 +5748,7 @@ StartupXLOG(void)
 		memcpy(page, endOfRecoveryInfo->lastPage, len);
 		memset(page + len, 0, XLOG_BLCKSZ - len);
 
-		XLogCtl->xlblocks[firstIdx] = endOfRecoveryInfo->lastPageBeginPtr + XLOG_BLCKSZ;
+		pg_atomic_write_u64(&XLogCtl->xlblocks[firstIdx], endOfRecoveryInfo->lastPageBeginPtr + XLOG_BLCKSZ);
 		XLogCtl->InitializedUpTo = endOfRecoveryInfo->lastPageBeginPtr + XLOG_BLCKSZ;
 	}
 	else